I'd say the fit on PP was about standard for age - we used to buy a lot of The Girl's clothes in France to avoid the any colour you like as long as it's pink rule of English shops and there would buy a size larger (French kids must be petite) but in PP I usually buy the age she is (or this year + next year if it's two years in one IYSWIM). The Girl bucks the family trend by being skinny.
If you can hang on for the end of the sale, the stuff is ridiculously cheap. Towards the end of the summer sale, I got The Girl two dresses for next summer for a total of £7 - they had been reduced to £7 each, but it was a BOGOF offer.
Have you also looked at M & Co? They get sneered at on S&B for being a bit naff (and their women's clothing is a bit elderly even for me) but they too have a lot of leggings and long tops which The Girl admires.