Bigger snails do eat more. If they are the right type, that may be the algae you want to get rid of. I had six large (1-1/2") turbos for a while after my 55g tank cycled. They cleaned up most of the algae very quickly. Some other small snails helped. The big ones continued to cause problems by knocking over pieces of coral or polyp rocks. I took them back to the LFS and am sticking to smaller ones along with my conchs. I know the conchs will continue to grow, but they seldom leave the sand bed.