Once when I was in college, a classmate who was tiny -- she must've been more than a head shorter than I was -- attacked me and I was toast. Turns out it doesn't matter how big the tickler is -- once she gets to your weak spots, you're at a disadvantage. (Though I do vividly remember, through the haze of years and my hysteria at the time, thinking that the sensation of her smaller-than-average hands scribbling away was particularly incapacitating -- it was like she could dodge my defenses and invade my space even more effectively than if she'd been a larger girl.)
The other difference? It seemed as though the spectacle of a woman half my size taking me down was even more hilarious than usual -- the friends who were looking on at the time were immensely amused, and seemed quite disappointed when she relented.