So many variables and options and all previews outside Develop differ from previews in that module.
AFAIK, there is little if any RAM overhead but again, much depends on the preview, size and what kind.
There can be quite a few including Minimal, Embedded & Sidecar, Standard, 1:1, Smart Previews etc.
With Minimal the thumbnail embedded in the document (raw) and will likely not match those LR creates from the data. It's tiny in size. But super fast to appear. LR must recreate them and does, this is why you'll often see these change in appearance after awhile.
Embedded & Sidecar extracts a JPEG preview of the raw; larger and faster than 1:1 but still not rendered from LR itself.
Standard is built by LR so it should visually match (within reason) to what you see in Develop. They are used in all modules expect Develop. So you might wish to concentrate on them and their possible sizes.
Smart Previews are intended to be used as a proxy when the original documents are offline. Low(er) resolution Lossy DNGs; partially processed raw data. They are smaller in size than most originals (there's a size limitation by design). So if you use them offline but zoom to 1:1, it's not the same as actually viewing the 1:1 previews so there are reasons for that preview too. If you're not doing off line work, you might consider ignoring Smart Previews.
1:1 previews are not used in Develop but in Library for quick zooming.
Standard size previews are also used in Library but not Develop (very briefly then not; that's where the cache comes in). The size of Standard size previews varies based on preferences. It's a pyramid like preview structure of JPEGs in Adobe RGB (1998). But 1:1 are full sized JPEGs in the same color space. Then there are the newer Smart Previews which are partially processed raw data, always at a fixed size regardless of the original raw.
