My new lover is huge. He is 6 foot 3 and weighs 300 pounds. He's so big that he crushes me when we have sex, and when I try to be on top, my legs barely fit around him. Have sex standing up, with you raised to a comfortable level on a pile of pillows. Or try getting it on when he is sitting in a chair, you on his lap facing him the change of angle might give your legs more leverage.
Sex can happen sideways in bed, nose to nose.
Experiment with doggy-style penetration, which has the added advantage of leaving his hands playfully free. Make love weightlessly in a whirlpool or deep bath. Browse how-to sex manuals. When it comes to sexual intercourse, where there's a will, there is more than one way. If the vast difference in your physiques really is a problem, it's one you can have fun solving.