Try a really tight band around the bottom... or take it to a place that can put poppers on it or if your handy sew some velcro on it.
Im having the same problem, my ds is in a 6-18mth gro bag and it is massive on him, he sleeps ok in it.. he is an a hammock and it flows over the end...
I havent tried any of the things suggested ... coz i have nt had it long and gro bags are expensive and i dont want to ruin it... but i think i'll give tying a band around a go!!!