Victoria's secret underwear?

ok so i'm starting to wonder if my underwear is too small for me..
i get size medium at victoria's secret
i'm 19 5'5 and weigh 125 so i'm a pretty average size.
i'm proportionate too, like i don't see why i would need large when i'm a pretty medium size.
but my underwear is always giving me a muffin top. it's tight and pushes my fat over it.
it's not uncomfortable at all. it feels fine, but it just kinda made me wonder today.. is underwear supposed to be tight? or is that not how it should be fitting?