Size and Weight
There's very little difference in terms of size between the D3100 and D3200. Of the two the D3100 is the slightly smaller, measuring 124x96x73mm compared to the D3200's 125x96x76mm dimensions.
Both the D3100 and D3200 accept SD media. Despite the marginal differences in size, both cameras weigh 505g with a battery and card inserted. As for power the D3100 uses a rechargeable Li-ion EN-EL14 battery whereas the D3200 is powered by the rechargeable Li-ion EN-EL5A battery.