The lower branch usually have smaller buds, because they are lower and the light don't reach them as the upper one, you should not worry about that. If you want them to get bigger you always put some CFL on the side of the plant or even an T5, may help get you more buds where the light not reaching.

For the fert you should check the manufacturer instructions... If you tell us what ure using we may be able to help you get better mix.