The whole game has to be loaded in RAM, usually mobile devices have a lot less RAM than desktop - and RAM is used by everything here, the filesystem, AGS, and everything else. Also there's a hard limit on RAM size allowed for the tab that simply depends - I think max it goes is 4GB in perfect temperature and pressure conditions. So reaaally bring down that number. Use sprite compression in AGS, and if possible use lower quality ogg audio, which may not be perceptible to the player and keep the game as nice as is. I think there was even a test for other sprite compression algorithms... Also reminder that the game size is full uncompressed game and not the zip you upload. Also remember that one person on a phone that is not warned before the download may end up spending all their limited data in your game by accident - itch.io warns all the time to keep size of the web game low, and this is one of the reasons.
About persistent savefiles, currently the implementation is a bit tricky, it works like this, the browser has a very limited storage per website, of around 50MB.
The idea is to sync the virtual filesystem save directory (which is mostly smokes and mirrors) to the key and value browser storage. Now the problem is that game savefiles in AGS can get quite big - the main culprit are Dynamic Sprites, if they exist when the game is saved, then we are going to serialize them, and they are serialized as bitmap, so they may take a lot if they are very big. So if you have lots of dynamic sprites you may need a custom save system.
The other thing is that if the user never access your game website after sometime, well, you will keep using that storage for your game in his computer indefinitely until they ever find out how to clean this in their browser. This is something that I don't know any easy solution. I wanted to add at least a sync option somewhere as a button so at least the person clicks this and this should remind them that they may need to delete this. The other possibility is to let people download their savegames, but this would not work with a custom save file. Anyway, there are some design issues I haven't been able to figure out.