My DD wasn't chunky but she went through a phase last school year when she was 4 where she only wanted to wear leggings. She still prefers leggings to most everything else but she will wear jeans this year thank goodness. She also refused to wear shorts this summer too.
C prefers elastic-waist jeans to ones with buttons. We have some nice ones from Gap and Crazy 8. The button ones fit him weird anyway--he has a big tummy but is otherwise on track size-wise for his age.
DS will not wear pants with buttons. He says "owie tummy! Owie!" So all his pants are slip-on's. Mostly cargos, but he has several jeans from TCP and Cherokee too.
This isn't a chunky kid thing. Many kids refuse to wear any button pants (jeans, cords, etc). My daughter wears leggings, yoga pants, fleece or sweat pants, "fake jeans" and other soft bottoms.
DS3 is only 17 months, but his tummy and thighs are still so big it's hard to find pants that work on him. He wears a lot of rompers right now...and look damn cute in them!