Lots of reasons, Some are more probable than others.
--Different variety
--Patchy fertilizer or patchy mixing of nitrogen rogbing unfinished compost
--Root damage/tight roots to smaller plants at transplant. Odd for three out of four
--Root damage/burn due to something you put in the planting holes.
--Vine borrer worms affecting smaller plants (would see a 1/8- 1/4 inch hole in the stem low down with what looks like vermiculite coming out of it.) Odd to have three out of four plants affected but it could happen.
My solution when I see something like that is usually to mix up some liquid fertilizer and pour it on the smaller plants to see if they catch up.
Thanks a lot for the responses. We'll give them more time. If the difference continues to increase, we'll try some fertilizer or other means to give the small ones a boost!