We aren't experts, so perhaps we'll eventually run into similar problems, but it seems like supporting a contributor to do something major might just be a case where it doesn't always make ROI on the first feature, but the point is to attract that contributor as a member of the core dev team, since every subsequent major improvement that contributor makes will become easier. We are trying the wizard avatars for significant contributions to try to motivate people to really join the Archmage team.

The other ingredient might be excitement to do things. People will spend more time figuring out how to make changes they care about if they're psyched; if they just want to fix some bug, they'll give up more quickly and need more time from you. So highlighting some cool things to do might help. I notice that you link to small tasks issues, but don't have any open right now. Always taking the time to write good issues that explain how to do a thing, even if you think you'll probably do it and aren't planning on doing it soon, is a good practice, because it'll be easier when you go to do it, and some contributor might see that and get inspired–then they'll know how to do it instead of needing to ask you where to start. Here's an example where I wrote up a lot of info on how to do a task and a contributor put in a lot more time to finish it afterward: https://github.com/codecombat/codecombat/issues/23