A static "attack" where the person is simply trying to ground out any movement that you are doing is plain and simple B.S.. Not only does this never happen in "real life" (you know, not in the dojo), but a static grab is simply not an attack.
I am also on the smaller side (5'5" on a good morning
) and have gotten to the point where I can execute technique on big people (One guy at our school had a nickname of "Vast"- 6'8" over 350lbs.). The first thing that you have to achieve is kazushi. When a person does not have dynamic equilibrium, the body automatically re-assigns resources to achieving that task, thereby "robbing" the person of usable strength. In order to achieve that, you really need to learn how to establish an energy connection (ki) with the person so as to move them from the inside out. Once that has been accomplished, techniques seem to suddenly work.
Until you get to that point, if a person is simply grounding you out in a static attack, give him some atemi "love" to establish kazushi