The effects of caffeine on muscle strength have been equivocal, and this is thought to be based on the muscle groups that have been evaluated.
A recent study looked maximal voluntary contraction strength in resistance-trained men randomized to testing with 6 mg/kg of caffeine (equivalent to about 3-4 cups of coffee) versus placebo.
The men who received caffeine experienced significantly increased muscle performance, according to a report in the Journal of Strength Conditioning Research (2014;28:3239-3244).
Both upper and lower body muscle groups were noted to improve with caffeine. While it seems that larger muscle groups may respond more favorably to caffeine, the influence on muscle group size remains in question.