Got the same problem here on both an iPad Pro 11” (2018) model as well as an iPad Pro 9.7”. Wish I had not upgraded to iPadOS 14.
I have reset the all settings on both iPad’s but all to no avail. Restarting the iPads only works for a few minutes before they start reverting to the random app icon size changing. If you change orientation of the iPad, then the small icons move to other apps. I have also found that if a folder is suffering from the size problem, then it also has odd sizes in it.
Even if I go to the settings and choose to reset to More or Bigger in Home Screen and Dock, it make no difference.
As the affected icons seem to change as you slide between pages or orientation, I can only guess it is an Apple bug that they did not catch. Hopefully this will be addressed in the next update, cos' the BS about them saying reinstall you iPad from factory is a load of crap. With over 950 apps on my iPad, I am not reinstalling them all, it takes a couple of days just to download, let alone set up. Then there is this rubbish about giving it permission to use the resources on my iPad, which reminds me of UAC in Windows Vista.
And finally a company the size and wealth of Apple, that is unable to solve simple problems on their own hardware is lamentable. Microsoft have that excuse, because they will often quote that they did not make the hardware they are being made to support, so with Apple that does not apply.
The screen shots show the problem clearly, with both More and Bigger settings, as well as folders.
Any takers