Big difference in first true leaves' size

seems a bit small but maybe related to the possible mutation you see... early leaves can have issues that you won't see in future growth. Don't worry too much about intiial problems as they may just go away. If they linger, that's when you might consider different options. Not enough info to fill in blanks. Could be too much light.. Could be a substrate that's slowing it down. At this stage, the pot should be tiny, so that is okay. Once those leaves are nearly as wide as the seedling pot, that's a good time to pot up. It's always a good idea to pop more than you need so you can kill off any laggards or defective plants. given the prices of some seed, i understand the consternation that may elicit. don't overpay for seeds :P