those sizes are pretty close. I wear both. Sometimes I order the guys' small if I the colors are different and I like the guys' better. THe girly shirt is just cut a little different.
In comparing on the size chart, ( http://threadless.com/sizechart ) guys small is W18 - L27.5. The girly Large is W17.75 - L27, so it's definitely close.
i just gave a girly Large to a boy and it fits him perfectly.. I think the arms are a little shorter for girls, but that's it..
there is one blog that compares the sizes, I will try and find the link
here it is: http://www.threadless.com/profile/595552/Mojie/blog/279210/Size_Comparisons_Girly_Small_Medium_Large_and_XL_tees_XS_S_and_M_Hoodies_BONUS_FOTL_Men_s_Small_tee_UPDATE_Now_with_washing_shrinkage
but i could be mistaken.