I think you can use ‘grow’, since there is an increase of’smallness’ compared to a previous state. If I grow larger, does it mean small fish turn small? From what we can tell, fish are slowing or they stop growing earlier. Nevertheless you have the act of ‘growing’ from tadpole to fish. This is what ‘grow’ refers to, it is not a general phenomenon that happens to the common fish..

So long and thanks for all the fish!