[racket] Why does drawing to pdf require a scaling factor of 1.25?

I found empirically that when I save a image to pdf I need to scale it by
1.25.

Code below.

Why?  Is there a setting I should use instead?


Thanks

Dan


#lang racket

(provide image->pdf)

(require racket/draw
         (only-in 2htdp/image
                  save-image
                  image-width
                  image-height
                  scale
                  ))

(define (image->bitmap image)
  (let* ([width (image-width image)]
         [height (image-height image)]
         [bm (make-bitmap width height)]
         [dc (send bm make-dc)])
    (send dc clear)
    (send image draw dc 0 0 0 0 width height 0 0 #f)
    bm))

(define (image->pdf image output-file)
  (define dc
    (new pdf-dc%
         [ interactive #f ]
         [ use-paper-bbox #f ]
         [ width (image-width image)]
         [ height (image-height image)]
         [ output output-file ]))

  (send* dc
    (start-doc "useless string")
    (start-page))

  (send dc draw-bitmap (image->bitmap (scale 1.25 image)) 0 0)

  (send* dc
    (end-page)
    (end-doc)))

(module+ test
  (require (only-in 2htdp/image
                    circle text square overlay above))

  (define im
    (overlay
     (above
      (circle 50 'solid 'red)
      (text "This is a red circle" 12 'black))
     (square 150 'solid 'lightblue)))

  (image->pdf im "image-test.pdf")
  (save-image im "image-test.png"))
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