How to de dup and remove imported thumbnails?

Hi I have imported about 250,000 images from my old iPhoto, Aperture and even Lightroom libraries. Very pleasant surprise how simple this is to do. It looks like I am getting three versions of every image, my guess being one original (full size file), one thumbnail (tiny file) and not sure about the third, maybe also a thumbnail. By no means all pics have this feature. 

What's a good way or ways to delete all these small images ? Quite happy to start over with generating thumbnails. 

If I had to guess I reckon it's 20 years of pics at 5,000 a year = 100,000 so 60% are surplus which approximately matches my guess that 2 of 3 are thumbnails. 

I can't say my experience of deplicating apps is good, they usually need careful attention to all duplicates found and with thes emany that's not practical. However, idnetificatiuon by file size might work is my hope.

In fact why I night call "actual duplicates" I don't mind, so that is where both files are full size and the image is the same. It's the "thumbnails" I want to get rid of by the tens of thousands.

My first post here so I hope it is in order, many thanks for any help.   

Lightroom Classic. Macos Big Sur - all latest updates.