Are there any downsides to getting very large frames?
Larger glasses with thick lenses will be HEAVY
The only downside is sometimes the glass has some distortion toward the edges. I love big frames but have had that problem. The glass is curved slightly and sometimes things look weird when your eyes shift to the side.
Heavier is one downside.
My prescription is -8.25 and, in my experience, the only real problem besides the shrinking/distortion is that the weight isn’t distributed as well with large frames. I’ve been using large frames for years, and I’ve noticed they tend to slip down my nose a lot. However, I just switched to plastic frames and I find them to be much lighter than the metal ones. They still slip more than the smaller framed ones would, but they aren’t as bulky. Honestly, the most important factor is lense quality, especially if your vision isn’t entirely corrected with glasses. Mine falls around 20/40 when I first get new lenses, but it usually ends up going down to 20/70 within a few months. The lense quality is a big help.
On a small face they just look goofy. Like a kid wearing Nana's glasses.
A large frame with that power just increases the edge thickness and creates more distortion when you look sideways. They won't do much to increase your peripheral vision , just give you more edge blur.