Congratulations. Yes, I too have experienced the problem of inconsistent sizing of Home Screen icons after updating to iPadOS14; I note also that the problem also extended to sizing of Widgets and the new Stack.
The obscure sizing was also strange in that some smaller icons would return to expected sizing - and others would shrink. Switching between Home screens caused different icons to resize on each of the Home Screens.
Having spend hours searching the updated iPad User Guide for details of this strange new feature (anticipating that the OS had some cunning new feature that was eluding logic) - I gave up - and force restarted my iPad; doing so appears to have resolved this particular bug.
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
I sincerely hope this helpfully resolves the problem for you too!