How come they fit me but not her?

yesterday my younger sister found a box in her closet full of her old underwear from when she was like 7-8 she said. well my sister is almost 14 now is really tall like my parents and from cheerleading i guess at 5'10"/118lbs. i just turned 17 and short thanks to my dumb aunts and grandmas genes at 4'8"/77lbs. i was in her closet when she found them and she really wanted to put them on again just to see, so i left while she tried a pair on when not even 10 seconds ped she came out to show me (i didnt expect her to actually show me.) but she started laughing saying her 's too big now for them and could barely get them up her thighs, so she asked me to put a pair on hoping theyd fit me. i of course said no way and walked away when she caught my attention by offering me $30 to do it. so i did, big mistake. yea in the end she did pay me $30 but if i woulda known how theyd look on me i woulda made it $500. first off they were actually alittle big on me which had her laughing so hard saying how she cant believe underwear she wore as an 8 year old was still big on me. how come the back of the underwear was so big on me? so me being all mad/embarred told her to shutup when then she took a pic of me sending it to all her friends. stupid me then told her to see if underwear i wore when i was 8 would be too big on her now, dont ask. i was so mortified i couldnt think of anything else so i actually went into my closet and found a similar box full of my old underwear, i gave her my batman briefs from when i was 9 telling her i used to wear it when i was 8 so she thought I was younger. she was so full of herself from feeling superior over me just cuz her old underwear was big on me. but yea bad idea making her put mine on, she said they were even smaller than her older underwear and she even showed me. you couldnt even make it out that batman was on the side cuz she stretched it all out. well whatever i know this is really random and i would never tell anyone else about this, but why does her old underwear fit me, and my old pair doesn't fit her at all? are her thighs/ that much bigger than mine? mean shes not fat at all and i could only think maybe cheerleading gives her somewhat more muscle in her legs than me.