Discarding 1:1 previews does not reduce size of preview folder

LR will only discard a 1:1 preview if its width is 2 or more times the width of your standard-sized previews; otherwise, LR keeps the 1:1 preview to use as the standard-sized preview.

So if your 1:1 previews are 4096 pixels, and your standard-sized previews are 2560, discarding the 1:1 previews will have no effect (since 4096 < 2 * 2560).

It's been a while since I've seen this come up:

https://www.lightroomqueen.com/community/threads/discarding-1-1-previews-doesnt-shrink-lightroom-catalog-previews-lrdata.31002/

This action isn't instantaneous. You should maybe let LR run overnight, but eventually, the previews will be discarded if requested.

I don't think the 1:1 previews are discarded IMMEDIATELY when you use the menu to discard them.  It used to take weeks to eventually remove them.  Maybe that is still the same.

Lightroom only builds a 1:1 preview if you zoom into the image 100%. By default it also discards 1:1 previews after one month. So most likely you did not have that many 1:1 previews to make a real difference in size.