It's fine to have smaller form of games. My previous jams are out there. You could try Rabbit Dimensions for example. It's something that I could make within 2 hours with my current skills. But it took me like 8 days to finish it last year.
As I recommended many people. You could try "Horizontal build" approach first. Which is to print out the name of the functions you wanted to make in that area, assuming you're not making a wave-based arena game like my Rabbit Dimensions which only need a few functions to perform.
Well even while I was developing Rabbit Dimensions, I was also print out stuffs that I need to work, for example, player's HP, or win condition.
Back when I was just starting with Unreal Engine 4, it's the game name "Ancient Gift" which caused me a lot of troubles just to modify the third person template into a first person game. It's full of bugs and the game took over 10 GB on my storage. Just the game alone was 4GB and in GDTV Jam 2021, we need to also submit the project folder which took nearly 7 GB. That's why I already deleted the game.
But hey, it's kinda functional enough and I was just start learning unreal from that jam.
The most noticeable achievement for me was the incentivize for me to reduce the project size as much as possible. Which allowed me to start working on finding the optimal workflow for UE4 and now my project sizes are roughly 600MB when I got a bunch of stuffs.
I'm not sure if my current jam project gonna be around 600MB though. I just hope that it won't exceed 1GB so that I could upload on itch without any problem.
I'm sure unity wouldn't have such project size problem since I used unity before I changed into unreal. I stick to unreal because everything is just superior :D