That really depends on the breed--there's a tremendous amount of variation in size, weight and build between breeds of cat. With kittens--as with any babies--your primary concern is that they're growing at an appropriate rate. As long as their weight is increasing at a consistent rate and not going up faster or slower than expected, there's no need to worry. Your vet is the best resource for questions regarding how fast you can expect your kittens to grow.
In the meantime, try to relax and enjoy your little fur-babies. :-)