No, just the reverse. She needs her legs wide apart so you can get in. She needs to push her butt up by curving her back.
Because you ar smaller and she is bigger any position where she is on the botton are going to be hard unless she is very limber. If she can get her legs over your shoulders - that would be a great position for you. Then you are not really on top - jut between her legs.
See the link where it says help keep this site free? Click on it - then click on the Susan's Sex Toy Shop link. (
http://www.sexsupport.org/toys.html ) Get some great lube called Eros, the cock rings, and get her a vibe too.
Also check my book list on AMAZON - there are two books about sex when one or both of you are bigger. These books happen to be good for people with disabilities too!
Susan