This is more for info. My girl also has thunder thighs. She was 8.6kg at her 4mth checkup. M size merries is already too tight for her now at 5mths. M size Charnis fits better but likely will still have to size up soon too. i have yet to try other brands.
Change to pants instead of diaper. Or change brand that has bigger cutting and can hold pee/poo without leaking. Like Merries, Huggies, Drypers
Size up or change brands. personally I feel Huggies is ang mo brand so cutting bigger. baby don't wear pants so I can't advise