And all the layouts have multiple tilemaps, but none of them exceed 2048x224 pixels.
Okay, well that's still over 2 million tiles, which is a pretty intensive case. I'd recommend keeping your tilemaps smaller to avoid wasting memory.
My project doesn't use a tilemap that big. I made the tilemap to exaggerate the issue so you could actually notice it. None of my tilemaps exceed 2048x224 pixels.
That's only 1,792 tiles so it shouldn't be an issue.
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So it's probably not that. The quickest way to identify it is to delete sections of your project away until the memory use goes down. That will help you narrow it down to a layout, layer or even a specific object that is taking up the most memory.
Apparently it was the max-height setting in the layout files being set to some ridiculous numbers like 40000 tiles and there's no way to change that unless you go into the files themselves with something like Notepad++. So I used regex find and replace to set the max-height to 14 rather than 40000 because none of my tilemaps ever are anything but 14 tiles high. Now the memory usage is 200mb. Maybe in the future you should consider making Construct 3 alter the max-width and max-height settings on the tilemaps to whatever size the tilemaps actually are?