Wouldn't the block size depend on how many transactions were in it when it was successfully mined? ie., you keep on adding transactions to it and as and when the hash generated is below the target hash, then this block is said to have been successfully created and a new block will start? So, determining the avg.size of the block based on the existing blocks would not provide a accurate number (or close to it) is what I think.

I'm a newbie here, so pls. correct me if my understanding is wrong.