Here's a link to an article about root growth on tomatoes. It's long but there's a lot of good information about what roots do that you don't see
http://www.soilandhealth.org/01aglibrary/010137veg.roots/010137ch26.html
repotting is the most important thing I've found in growing tomatos indoors. I had them root bound in a small newspaper seed cup. I have another post on here about container tomatoes being root bound, this happened this year.https://www.backyardchickens.com/t/900510/root-bound-container-tomato