What is the largest RAM expansion you've seen for Amiga or Commodore?
Big box Amigas (A3000/A4000) with Zorro-III 32-bit bus can accommodate 256MB RAM expansion cards, which are still being manufactured today. Utilizing 4 Zorro-III slots, this allows memory expansion of 1GB, which is quite substantial for a 68030/40/60 based computer. I'm not sure how you'd use it all, excepting some bespoke application that you could write yourself.
Small Amigas such as the A1200 usually only acquire large expansion memories when that RAM is fitted to an accelerator card. Most of those cards "only" support up to 128 MB. Still, a very substantial amount of memory on an Amiga.
The "standard" large memory expansion for the 8-bit Commodore machines is expansion cards that mimic the original Commodore Ram Expansion Unit ("REU"). First released alongside the C128, the REU allowed a C128 to be expanded to 640KB by providing up to 512KB, or 8 banks of 64KB. It is also compatible with the C64. However, the way the REU functions means it can support up to 256 banks of 64KB, or 16MB of expansion memory. Compatible REU's of this size are still manufactured today. You can use such "huge" REU's as RAM disks, and I know some demo scene productions have made use of them to create full-motion video.