While this is not as distressing as getting no fruit set, it can still be a serious problem. Fruit should be in the 8 to 10 ounce size for standard (beefsteak) greenhouse varieties. Possible causes include 1) poor pollination - be sure you are pollinating every other day if doing it by hand (those greenhouses using bumblebees should not have this problem, unless the bees are dead). If this is the cause, the seed count will be low. You can check this yourself by cutting the fruit transversely (left to right) and seeing if the locules are full of seeds or if seeds are scarce. 2) lack of water (wilting) can cut down on fruit development. 3) high temperature stress will limit fruit size. This is very common in the hot, humid southeast, especially in May and June. 4) shading during the cloudier months prevents full development of fruit. Be sure the greenhouse does not get shaded by large trees or tall buildings.