Help with random desktop icons changing to a smaller size.

Hello,
I'd like help with a minor but annoying problem I've had for a few years.
I like to make my own desktop icons for sites such as Radio 2 , Discogs, BBC Sport etc... the sizes are 256 x 256 including a white border
on most of them (as seen in the screenshot)
I keep them all in a folder in Data (D:) I assume this is a suitable location for them.
Every now and then when I switch my PC on, some of the icons have become smaller (usually about a dozen or so)
I easily restore them back to a normal size by just renaming the icons, but it is a chore to keep doing this.

Does anyone know what causes this?
It always used to happen in Windows 7 and I've just moved straight to a Windows 11 PC , but still have the same annoying problem.
Thanks for any help.

Are you saving these as icon file formats?
.ico
What type of file are you starting out with?
A jpg or a png will turn out looking different than the others.

I've always made them with irfanview. I usually convert jpg images to ICO Windows Icon
I could always try the online converter to just replace a few and monitor them to see if they become smaller over time.
There are about 40 icons I regularly use, so it would be time consuming to replace them all in one go.
Worth doing if it fixed the problem though I suppose.
I thought it could be something to do with iconcache or is that not connected to the desktop icons for website URLs?

I had issues using irfanview also for creating icons. I tried the online website and the ones I got worked. It also does batch conversion so you could do 40 at once...