Well, "Size of the game" and "Wear on the ol' engine" is two different things
You can technically cram TB's worth of png files into the game-- it's moreso just rendering multiple objects/multiple NPC's/goofy effects where the engine really blows a gasket. I don't think our precious time with the seaweed girl is going to be interrupted by the 300 other questionably fuckable fauna.
Unless you pull some weird png orgy like other games that give you less than a second to see the art before spamming another variant irreversibly, not naming
anyone in general
Normally, you'd be really right, but it's the RPG Maker Engines we're talking about here (especially the old ones). Due to possibly them being unoptimized or some other backend issues on them, they lag way faster as you try to add more and more Entries. It could possibly be on how the engines (the old ones, including the VX Ace one) handle its Database, possibly having the Database pre-loaded or loaded more than needed.
A test of this situation you can do, if you wish, by opening a random (smaller) VX Ace game in the Editor (you can open any game in the Editor by copying the "Game.rvproj2" project file (of any Project) into the game's folder. It should load the game up (including its database etc) fairly fast.
Now, do the same thing with MGQP. In my own experience (and this on a decent PC, capable of running modern AAA games, so no excuses for an VX Ace game), you'll have an extremely slow time doing this, with just the Current Part 1+2 game.
Now I'll admit, I'm one to notice this Lag issue moreso because I like modding my games, so I did try to touch this game with various tools (including the Editor, MTool, Cheat Engine for Speedhack etc.). Specifically using the Speedhack since I almost always feel like RPG Maker games are way slower (in various animations) than they should be. (On this note, I did Mod MGQP to be able to get fair Exp scaling according to the Difficulty chosen).
Still, already the Engine is creaking at the seams with having such a ludicrous amount of Entries in the Database, especially for a VX Ace game (check it out, I was seeing, even in a random category such as Items, Weapons or Common Events, up to 5000~ Entries, when most games barely breach 200-600 per category). If we're to believe Part 3 adds 150% more content, I feel like we'll truly have the game goddamn crash upon opening the TitleScreen, regardless of our own Gaming Rig.
In all honesty, VX Ace wasn't the right call for such an immense game. Even switching to just MV would've made a world of difference, in terms of overall Performance and capabilities. I'm guessing they went for VX Ace due to familiarity with the old engine (at the time they started development on MGQP), and as they most likely weren't fully aware of the sheer size of the final product they'd end up having, within that engine.
Still, if it works, it works.... But holly hell, that would be a miracle surpassing Ilias of all the Worlds.