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There are two different types of muscular hypertrophy: sarcoplasmic and myofibrillar (muscles also increase in size due to a small amount of hyperplasia but this contribution is minimal). During sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength. During myofibrillar hypertrophy, the myofibrils, being the actin and myosin contractile proteins, increase in number and add to muscular strength as well as a small increase in the size of the muscle. Sarcoplasmic hypertrophy is characteristic of the muscles of bodybuilders while myofibrillar hypertrophy is characteristic of weightlifters.[4]
What muscle builders refer to as a toned physique is one that combines reasonable muscular size with moderate levels of body fat, qualities that may result from a combination of diet and exercise. High-repetition exercises of 8-12 reps indeed do cause hypertrophy of the sarcoplasm in slow-twitch and high-twitch muscle fibers, contributing to overall increased muscle bulk. This is not to be confused with myofibril hypertrophy which leads to lifting gains. Both however can occur to an extent during this rep range.
http://en.wikipedia.org/wiki/Strength_training
http://en.wikipedia.org/wiki/Sarcoplasm
You can look in to the references if you want. Guaranteed those scientists know more about it than you or your personal trainer.
awesome post. +k