Cumulus clouds develop as follows: unsaturated air heats at ground, becomes bouyant, and rises in updrafts - cooling until it reaches the point condensation. Air over a given locale is usually pretty well mixed, such that the temperature and dewpoint of the updrafts is very similar. Thus they condense consistently very near the same level - that of the rather flat bases. Once a flat base is formed, air directly below that is very nearly saturtated. So any further air moition upward reinforces the base, and any horizontal air motion near the base and subsequent rising will tend to cause the cloud base to expand at the same height.
The top & sides are fluffy because the bouyant plumes of air encounter warmer & drier air at some point, they rise slower and eventually stop, and the condensation ceases. Downdrafts also develop at the periphery of cumulus clouds.