As a new aikido student and lifelong widebody, I can tell you that your size makes a difference only as long as you let it. It took me years to work up the guts to try aikido, and I daily kick myself for not starting sooner. I don't know if this is universal, but in my case I am:
2)gaining coordination (I think
3)gaining energy (the more I do aikido, the more I want to do aikido)
Bottom line, your sensei is right, don't worry about what other people might think. Train, have fun, invest in ibuprofen.
My 2 cents.