Several things can affect the size and makeup of your muscles:
- What you eat. Eating carbs post workout can help increase the size of the muscles for a time. Also, more fat in your muscle tissue adds to the bulk. If you burn that fat, the muscle appears to shrink even though the strength is unaffected.
- How you train. Long endurance work makes muscles that are smaller in size. Even differences in how you do weight lifting can affect the size and strength of the muscle.
- Stress. Stress is catabolic, and too much stress over a prolonged period of time will induce overtraining and loss of muscle.
- Lack of training. Muscles can atrophy if they aren't used. However, this takes weeks and months.
All that said, you will have some variance in muscle size due to the fact that they are living tissue, and the fact that there is always a margin of error in measurement. I've noticed that my calves and thighs can vary fairly widely (.5 inch) from week to week. I'm mostly interested in strength, so for me this is simply anecdotal.