can you delete an iphotos migrated library?

Yes. However, make sure the new Photos library does have all of the items in the migrated library before deleting it. It's often recommended that one store the library on an external HD for a couple of months just in case they need to recover any of the original files in it.

Also be aware that you won't recover the full apparent size of the library as shown by the Finder because the original files share the same space as the original files in the new Photos library and all you'll recover is the storage that the other files in the iPhoto library took up.