The prose is functional; I think the biggest nitpick I have is the repetition of some phrases and words. In terms of pacing, the game patient but not slow, taking its time to set up properly and managing to imply enough history to make the developments feel earned. I also like that the comedy inherent to the premise goes so understated – it's all pleasantly mundane in spite of involving a wacky magic gem with inexplicable powers.
That being said... given that there's a not-insignificant amount of words to read and that the plot leans towards thin, with the mechanics and the mysteries of the whole thing being so vague, I feel like Expanding Sizes and Feelings could have safely ended on a more conclusive note. I'm not even necessarily talking about resolving every plot thread; just something more satisfying as an emotional ending would be enough.
The jam theme is applied in a bluntly literal but effective way. The only thing I'm not quite sure about is the "and" in the title – I feel like the metaphorical link between physical and mental expansion would be the most interesting thing for a story like this to explore, and there's not necessarily a lot of that in here.