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When I got my 2 RES about 4 months ago, they were both about one and a half inches long, (one slightly bigger). I had trouble getting the smaller one to eat for the first 6 weeks or so, but now they both eat well. The thing is, while the larger one has just about doubled in size, the other one hasn't grown at all. I monitor them to make sure they are both getting plenty of food (pellets, brine shrimp and a variety of veggies all mashed up together) but it doesn't seem to help.
Is this difference in growth rate normal? Is the big guy growing abnormally fast, or the little guy abnormally slowly?
They are in a 10 gallon tank right now with proper heating/lighting/basking area/filter. Thanks for any input.

It's easy for one turtle to grab more food than the other. Best way to do it is to measure the food and feed them separately. You may want to cut down the protein for the larger turt (they shouldn't be getting "plenty").

Its too early for you to tell the gender, but a female will grow larger than a male and in very rare cases, an illness or genetics will inhibit growth.