Very few questions are more posed to trainers and occur to more people than that of whether certain training will result in tone or bulk. Most trainers, though well aware that building huge muscles is a lot easier said than done, still have a hard time convincing clients of this fact under the current bombardment of information. This paradox between trainer and media potentially leads to inaction for people which is the worst case scenario.
The fact of the matter is that whether your weight-training is going to "tone you up" or "bulk you up" is dependent upon genetics and the circumstances surrounding the training period. For most, no amount of weight-training and eating is going to result in anything even close to immense muscle size because such a trait wasn't very successful during human evolution and, therefore, not a trait commonly expressed. Secondly, with heredity on your side or not, diet is a critical factor in determining the degree of tone/bulk you acquire. In addition to diet/genetics, stresses outside the weight-room, and recent training practices can greatly impact whether or not you'll tone-up/bulk
WHAT IS TONE VS BULK?
Now before we get into how one "tones" or "bulks" up, it's helpful for trainers and clients to have an understanding of what it usually means to "tone up" and "bulk up". The physiological definition of tone has to do with the tension characterizing a muscle (hypertonicity/hypotonicity); however for purposes here, the prevailing lay perception of tone/bulk will be referenced. Usually "toning up" and "bulking up" both involve making muscles appear more noticeable, via increased size and/or definition.
The primary difference between toning and bulking is often why the muscles appear more noticeable ( Again, this is based on lay convention, which varies). Toned muscles appear more noticeable because of less fat covering them without much hypertrophy, while bulked muscles appear more noticeable because of increased size. As such, more noticeable muscles can be acquired by losing fat while retaining as much muscle as possible (tone), losing fat while gaining muscle (tone and or bulk depending on beholder), gaining weight that's predominantly muscle (bulk), and gaining both fat and muscle that, overall, makes you look "more-muscular" (prototypical bulk).
ROLE OF EVOLUTION
It's likely human evolution has made building large muscles difficult because, for most of human existence, the circumstances and amount of food necessary to support muscular populations wasn't available. Early populations that were more muscular would have required more food. Such an increased need for food, during frequent famine, decreased their chances of living long enough to pass on those "muscular-genes". Therefore, the expression of muscular genes isn't nearly as common as genes of other body types.
To the concerned women out there: no amount of heavy lifting is going to result in “bulk” without a lot of food; if lifting heavy was all that was necessary to “bulk-up”, we’d provide gyms for starving regions rather than food.
Diet effects muscle definition in that the amount of materials and energy a diet provides can support or reverse fat-loss (tone)/muscle-gain (bulk). Diets sufficient in nutrients to support heavy-training but "just-short" of maintainence calories can allow for better tone via muscle maintainence while losing fat and weight. Furtheremore, a diet that's sufficient in nutrients while providing maintainence calories, can maintain body-weight and make muscles more noticeable via increased size without fat-gain; whether such change constitutes tone/bulk is subjective. Purely on the bulking front, a diet that provides more than enough nutrients and energy to "supercompensate" heavier/more-demanding training can result in a relatively large amount of "bulk" gained*. Conversely, if one's diet is deficient in nutrients/energy, heavier/more-demanding training might result in muscle loss which can result in a smaller but "softer" appearance (neither tone nor bulk).
*If recovery is also sufficient.
Stresses outside of the weight-room that can impair both tone/bulk include lack of sleep, illness, and too much activity. Impaired sleep blunts muscle definition by not allowing the nervous system/muscles to rest and altering hormone levels in a way that encourages muscle wasting and fat gain. Illness compromises muscle definition by hording nutrition for immunity and raising cortisol, leaving muscles under-nourished and wasting, while aiding fat gain. Too much activity, in addition to hording nutrition, also alters hormone levels in a way that can result in a muscle wasting fat-gain state.
The degree a given training regimen preserves/adds muscle is also influenced by the training that preceded it. If a previous training period was much more depleting than the one current, the likelihood of the current training period allowing for "catch-up" growth is increased. When the most recent training period demanded much less repair than the one current, muscle gain can either be more or less likely. If the recent training period allowed for “catch-up” hypertrophy, then further hypertrophy is less likely. If the previous training period was light long-past the point of “catch-up” hypertrophy, a current training period that is sensibly more intense is more likely to be accompanied by hypertrophy. During the periods in which damage rate exceeds recovery rate, however, muscle-loss as well as fat-gain can occur resulting in less definition. Essentially muscle gain depends upon how much REPAIRABLE damage occurs, which means that, during highly stressfull periods, less disruptive training might be all that the body can tolerate in order to maintain /add muscle. Conversely, more disruptive training, during stressfull periods, might "shrink" muscles and decrease tone/bulk .
THE BOTTOM LINE
The take home message is that whether your training wil have you “tone-up” or "bulk-up" depends on the degree you can repair training damage. If you're genetically-gifted, chemically assisted, eat 10,000 calories/day, and sleep more than you're awake, you can become more defined/toned via gaining a lot of muscle by repairing a lot of training damage with little or no fat-gain. Those of us who work for a living however, might have to be extremely diligent about keeping training damage to a manageable level in order to avoid looking even smaller and softer for our efforts. This means that if you want more defined muscles via tone or bulk, nutrient/energy intake, stress, and training should all be in correct proportion*.
*Even with a lot of recovery/nutrition, training damage can be excessive resulting in muscle loss.