I don't know if the logic here actually adds up, and I also have no idea how you'd test it. It's not about wearing things that make you appear bigger, it's about suddenly having reduced density with increased volume while maintaining equal strength, weight, and mobility. I don't think it's something that can truly be replicated in an actual physical universe.
Which, given that is the case, let's just say it does make sense. HOORAY!
Yes This is Actually Game Commentary
Something I forgot to mention in the previous commentary is that I didn't just make up weight potions myself. There are actual weight-related potions in Fable 2 added by one of the previously mentioned DLCs, and they are the fastest way in the game to either gain or lose weight.
Previously, the fastest way to gain was via pies, and the fastest way to lose was via celery. If I recall correctly, pie could be as much as +15 weight per pie, and celery was at most -5 weight per stick, so the potions were far more of a big deal for quick weight loss than gain. Nonetheless, there were weight gain potions that were even more immediate in their effects than pies.
Also, while I think height does affect movement speed, weight doesn't. You can have the most or lowest weight possible, and beyond affecting some NPC interactions, it has no effect on gameplay beyond cosmetics.
Height + Speed?
I haven't read that height affects movement speed in Fable 2 anywhere, but I've played around with changing sizes in Bethesda RPG games. If you become bigger while moving at the same speed as before, it feels slow. If you become smaller and move at the same speed as before, you feel faster.
The speed feels constant to me in Fable 2, but you do get bigger as you level up certain things, which suggests to me you are moving faster even if it doesn't feel that way. If you didn't, being tall would make things feel sluggish, and I don't recall that happening.