My Japanese eggplant didn't start growing until the past 3 weeks. I started seeds back in early February and the eggplant stayed about 3 inches tall until just recently. It has grown a foot in the past 2 weeks, I swear...
Eggplants LOVE the heat! They will stay smaller until you're uncomfortably warm while tending your garden, lol. At least that's what happens in my experience.