I think with the bigger pots they spend more time early on making roots than plant. Give it a few more days and see if they don't turn a corner and start packing on the size. If I'm reading correctly you aren't even at day 21 yet? They look good for just a couple weeks.
Yeah their quite young. I'm just worried that I can't get 1 happy when the others are thriving. I know pheno types develop differently but I thought there reaction to environment would all be the same