I think another way to interpret this is to make a prototype for large projects. Do you agree?

Yes, then get some feedback and evaluate if it is worth to create it as a full game!

In your previous projects do you think the problem is that you underestimated the size/complexity of the project before coding

That's often the case

Why did progress slow down

The more code you have, the more the project becomes complex. At the beginning you are pumping features and mechanics daily, but after a few months you spend weeks bug fixing, this deteriorates your motivation. It is important to avoid technical debt!