Like others have said already, there is a 1GB size limit to projects but you can increase it on request. Most people just don't know that though.
I wasn't able to upload my first project on itch due to it being near 3GB. I instead went to game jolt as they didn't have that restriction.
After several months, I decided it was time for optimization. I removed some unused levels, modes, textures, items e.t.c, but it was just a little over the 1GB limit. I contacted itch's support through email and asked them if there was any way to increase the limit. They increased it to 2GB for me.
Now it has been upgraded to 4GB limit. Not sure why, cause my projects are now around 900mb, maybe because I'm using butler now and previously requested for a size increase but I certainly appreciate it.