Ebooks are tiny until the library manager converts them

A text ebook is a megabyte or two, so a library of a thousand books is a couple of gigabytes at most. What changes that is conversion and comics: a manager keeping several formats per book, and any illustrated title, which is images rather than text.

Where each app keeps its library

Get-ChildItem "$env:USERPROFILE\Calibre Library","$env:USERPROFILE\Documents\Calibre Library","$env:LOCALAPPDATA\Amazon\Kindle","$env:USERPROFILE\Documents\My Kindle Content" -EA SilentlyContinue |
  ForEach-Object {
    $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
    '{0,8:N0} MB  {1}' -f ($s/1MB), $_.FullName
  }

Converted formats are the multiplier

A manager that converts on import ends up with the original plus each format generated, per book. Three formats for a thousand books is three libraries. Calibre's own interface lists formats per book and can remove all but one in bulk, which is the tidy fix.

Comics and illustrated books

Get-ChildItem "$env:USERPROFILE\Calibre Library" -Recurse -File -EA SilentlyContinue |
  Where-Object Length -gt 50MB |
  Sort-Object Length -Descending |
  Select-Object @{n='MB';e={[math]::Round($_.Length/1MB)}}, Name -First 15

A comic or a photography book is a folder of images in a container, and a single volume can exceed a hundred megabytes. On a library where those are a minority of titles they are usually the majority of the size.

Downloaded copies in reading apps

Reading apps download books and keep them. Removing a download keeps the book in your account, which is the reversible option, and it is the same distinction as everywhere else in this category.

Backing up rather than pruning

A library of purchased or converted ebooks with your own annotations is small enough to fit on any drive and genuinely hard to reassemble. Moving it rather than pruning it is usually the better answer.