I need help I don't want to offend my mother in law but I'm not sure about the rocker she bought for him?

to use while we are visitting there. He is a big boy, over 4 months and already 18 pounds. I am afraid it ewill be too small and Ill have to tell her. She isnt all up on babies anymore, theres a lot she just doesnt seem to know. And I dont want my baby sleeping in something so tight or could possible fall out of. he rolls and moves around a lot now! Help me think of a way to tell her, if its small, how can I not hurt her feelings?? or pre warn her that he may not sleep in it??