Hi I currently have about 20,000 photos in I photo, which the 'info' displays as being 68GB's worth. However the I Photo Library with the Pictures Folder displays it's size as 341.21GB ... Why would that be? How do I look at what's in the I Photo Library?

This is normal and there is nothing to delete.

The iPhoto Window reports the size of the resulting folder if you exported everything at the 'Current' setting.

The iPhoto Library in Pictures contains:

Your original photos and videos, thumbnails, metadata and cache files, previews and so on. All of it is required for iPhoto to work.

Best to keep the terms clear.

You export photos from iPhoto.

You move the iPhoto Library to an external disk.

They're are quite different processes. Exporting means at the end you have a file outside iPhoto. Moving means the files are still inside iPhoto.

So if you say:

I'm currently exporting the library to an external drive

That reads like I''ve selected all the photos in iPhoto and am exporting them to a folder on this external drive, and at the end I'll have a folder full of photos outside iPhoto. Is that what you mean?

Is it best to have a 'master' I photo library on an external drive for all the thousands of pics Im never really going to look at often - and a smaller one kept on the HD for day to day usage ...

Best? Hard to know. It might make sense on a portable. But if you're on an iMac why do you need a smaller one on the internal? Why not just have the whole library on the external? There is no performace hit.