Best fit vs actual size!
Best fit I would think would work on aspect ratio if your image is 4:3 aspect ratio but your viewer is 4:4 etc it has to adjust and this might well cause the zoom.
So at a "guess" why not make your viewing window the "same" aspect ratio of the still images. Can't hurt to test the theory!
Aspect ratio is the width vs height of the image in percentage. Match the viewer to the same percentage values.
Or: Using Photoshop's "Rectangular Marquee Tool" in Style "Fixed Size" enter the exact dimensions of your existing viewer window and crop the image down and try using "actual size".