The thing I found with life jackets, I think my girl "fit" an xs or xxs too, but had to go a size or two higher because in spite of the girth fitting as she's skinny, the length didn't work out.
Uni is probably a little under an inch longer than my girl. The thing is, when she gets tired from swimming, do you notice her lower half starting to "sink"? So with the life jacket being the right length to cover more of her back, I think that's pretty important for it to make swimming a breeze for them. Of course, I never got the chance to put it to the test because my girl, unfortunately, has a deathly fear of water in most forms.
