The 8a is 3mm wider, 8mm taller, and 45g heavier. Perhaps that's not a huge difference, but I already consider the 4a too big. I will likely continue not upgrading as long as LineageOS and the hardware remain usable.
I switched from 4a to 8a. The weight difference is substantial and really noticable at first. I've gotten used to it now and it runs great with GrapheneOS, but it marked an end of an era of smaller, lighter phones for me.
FWIW I was on Pixel 3a until last month; bought Pixel 6 in the meantime, but it was too big and heavy to carry around, so I only used it as a "tablet", only at home.
Just updated to Pixel 8 and it fits the hand very well (including Spigen Liquid Air cover), and doesn't feel too heavy. Have a few friends who got Pixel 8 last month on sale as well and they all confirm.