Try different brands - there is no consistency in sizing. My 5 year old wore an 18 month size coat for 3 years - even last year when his shirts were a 5T. This summer he wore some shorts that were size 12 months or 18 months, and his pant size is 3T.
If you really can't find pants that fit, you might need to roll up the pant legs (or hem them if you sew). I have a skinny kid - so I always try to get pants with an adjustable waistband as well - otherwise I need to use a belt.