The size of a game comes down to its assets not how much gameplay it has
Models textures and sound files are what costs the GBs the data for quests and such never amont to much data size wise
5GB for a demo is because the demo has a pretty decent enviroment vs it being a blank open field or void
all those enviroment models and textures are what costs 5GB, if this was a more photorealistic game with this enviroment you can imagen this costing more in data size with more highres textures.
The anime artstyle allows for alot lower res textures as the level of detail is alot less
Optimisation is typicaly done later in development, after all core features are added. You cant really optimise something that isnt finisehd being expanded or you will just be redoing the same optimisation things over and over. Which for i assume a single dev is abit of a waste of time. There are some things that can be optimised which will likely be done in the next few builds
Something to note, the performance of a game can differ when in the editor vs a built build of a game.
I have used the ue editor and it can have better perfomance in the editor then when its compiled and can defonatly catch you out
Tbh for a 0.01 build the quality is way higher then many other 0.01 builds here and in general
Atleast visualy speaking, it obviously needs a few more builds to be a playable thing but this is not meant to be a playable thing its a player test to see if there are any majour issues to resolve early such as unforseen crashes and majour bugs(such as the animation arm issue posted a few posts back)