Toddler vs little girls sizing HELP MEEE

Ok so if a XS in little girls is a 4/5 is that like a 4t/5T with a little different fit like longer and leaner? Or what is the difference?! Struggling to figure out what size my girl should be in, I don’t know if she needs to go into girls XS or S

Examples: she’s been in a 5T but can fit little girls XS in shirts (haven’t tried pants but we struggle with pants being too big in the waist but not long enough in smaller sizes) and she is a 4 in a little girls underwear… her underwear is getting small, her shirts are looking baggy (losing toddler chubs) but she needs the length, her 5T swimsuit fits but is riding up way more so I don’t think it’s long enough