I too am small & what I have found is that it works better for me to actually squat... get on top & instead of being on your knees squat up & down. You can go slow, fast, easy or more aggressive. Have your partner put his hands on your waist for a little support... plus he can guide you as to how he'd like it to go (little faster or harder, you get the picture). Or you may want to try being on one knee & on one foot (you get a little more leverage without tiring as quickly as squating)... sounds a little strange but if your partner will tilt slightly to the side you are on your knee it works beautifully... I like to put my hand on/under his hip (on the side I am on foot) and kind of guide him as to how much motion I want him to make or how much tilt I want... trust me, it works & is a cool change (we try lots of stuff) Also if you are on your knees try tilting your pelvis at different angles... sometimes that can make a difference..and like Andrew said you may just want to let him be in control until you feel comfortable.
Keep trying things till you find what works best.