I appreciate that the Stamp Community Forum is primarily made up of stamp collectors, and I am occasionally amazed at the dedication shown by some of the advanced members in terms of size of their collections, number of albums, and making pages, and remounting collections from time to time, etc.
However, we postal history collectors also know how to accumulate and organize collections of covers -- which are certainly more bulky than those tiny little pieces of paper.
Of course, I still have my 7 volume Big Blue set to 1960, and a number of US and Israel albums, and something like 40 Avery 3-ring Binders with my better cover materials on Vario sheets, but the bulk of my collection is in more common postal covers. My focus at one time wwas to compile a reference collection of all 20th (and now 21st) century postal markings on cover from all Texas post offices, together with what I could afford from the 19th century Texas field.
I currently have THREE large Card File cabinets designed for 5" x 8" file cards, two with 14 drawers and one with 10 drawers, each relatively full (each drawer mostly filled, with some space for expansion at the back of each drawer - full capacity maybe 2000 postal cards or 1500 covers PER DRAWER.) I have another 10 drawer 4" x 6" file card cabinet which is suitable for postal cards which is slowly filling up and I move things around.
Since these file cabinets don't accommodate legal size (No 10) envelopes, I have several dozen shoe boxes, envelopes boxes and the like for these, And larger boxes with unsorted and un-filed covers for the proverbial "rainy day".
And NO, I have no idea just how many covers there might be in all this.

And this doesn't inlcude the multiple bookcases and ordinary file cabinets with reference materials.
And, to help explain it all, no wife to tell me I have too much "stuff" -- although occasionally my son steps in and points this out!
Mike