No comparison here I'm afraid as I have big boys - both were 2 stone when they turned 2 but I am at the smalls still fitting but at least you'll get good use out of them!
I don't think Jake would ever have fitted into smalls But Bella was tiny as well and was still in the upper end of small/lower end of mediums when she was about 1.
we use smalls on my 9lb little man oakley. and i use mediums on my 24lb mr T. tabs meet in the middle. we like mediums, they fit both boys! but generally we use them indoors for running round in nappy and a t shirt.the larges fit t with crossed tabs though