Someone Explain Underwear to me Please

Underwear and Swimsuits really. In the past month I have purchased underwear for all 3 of my kiddo's. And 2 out of 3 have been super simple.

The Boys of course are the simple ones............ah.......that isn't exactly meant to be a slam on the male gender......although it is kinda funny when you write it out like that - then again they are my sons and my understandning is a Mother's bond with her Boy Children is supposed to be much less teasing?

Anyway - it worked like this:
Spidey - wears a size 8/10 pants - his underwear are a size 10 (because I wanted there to be plent of time to wear them so I bought them on the larger end - pun intended).
TheKing - wears a size 4T - his underwear are a size 4T
SuperStar wears a size 10/12 - I bought her 10/12 underwear. They didn't fit. She said it was the cut, rode up her tushie. So I took them back. Bought a brief style underewear in 10/12. They didn't fit. Thought about my own underwear (like you want to think about them too) and realized they are an 8 when I wear a 10 - so one size down. Went back to the store........bought a size down for her. They didn't fit. Took them back - the lady now recognizes me - and bought a 14/16 size. They totally fit, a bit baggy, but time and shrinking (they are cotton after all) will prove to make them perfect.

The whole scenario made me wonder what the deal is with girls and women's underwear - and swimming suits really too. I mean seriously - shopping for a swimsuit is really equal to torture. The fact that you have to EXPECT to purchase a suit that is 1 to 2 sizes larger then you already wear (which makes some women cry) is just mean. Why don't the makers of the suits just tag them the size them woman trying them on will be? I mean if I wear a 10 then I should fit in suits that are a size 10 - I should not have to try on a 14 - or because I have ample boobage have to try on an even larger top and hope that the store will allow me to purchase a smaller bottom.

Sheesh - is this all to ding our self-esteem so that we purchase diet products?