What is the explanation for the fact that most cells are small and have cell membranes with many convolutions?

cells are small in order to maintain a good size to volume ratio which is very important in maintaining efficient metabolic processes in the body. I assume that you are referring to the crystae (the flagella like thingies in the cell wall) or the inner mitochondrion layer (which has pretty much the same structure) when you say "convolutions". Well this is still tied back to maintaining that efficient size to volume ratio. Cells want to store the max amount of things in a minimum amount of space. Like when I pack my bags before I go to vacation. I just don't dump everything in a disorganized heap into my bag. Otherwise it would not close (I pack a lot of stuff xD) Instead, I will and fold my clothes, arrange my affections, etc. in order to maximize the use of every inch of space in my bag so that it will fit. this is the same case with cells : p