How Much Difference Will 100cc Make for Someone Small?

This will probably cause an increase from 2/3 of a cup to 3/4 cup in your existing size (guesitmate).

A 100 cc increase in size all depends upon the size you are now and your overall body habitus.  It is a bigger change if you  only have 200 cc implants vs 500 cc implants.

A 100cc implant is essentially the smallest saline implant made and will increase your own B cup by approximately one half cup size. This is a relatively subtle change for almost any frame. It is hard to give definitive advice without actually seeing you but I would I council you to go for it!

Plastic surgeons still have a problem measuring breast volume and what effect a given volume will have on a breast. The breast is a complicated three-dimensional shape and size is not pure volume. It's also shape and location of volume. 

In general, an average width breast with properly fitted medium profile implants will be around 350 cc and the effect will be to increase about 1 1/2 cup sizes. 100 cc's up or down from this (high or low profile) will make the breast look about 1/2 cup larger or smaller. 

Most breast size asymmetries are not a full half cup but if that's what it seems to be than 100 cc's would offset it. More likely there are shape or position differences that make it hard to judge. Another thing that doesn't work well is using sizers and sitting up the patient during surgery. It's best to identify exactly how one breast is different from the other and accept the shape or position differences and then choose a volume difference based on experience.