As mentioned a screenshot helps.
But I guess you mean, if an image has a different aspect ratio than the Previews window, the window doesn't resize itself to the image but fills the "empty" space with grey? Like vertical "letterboxing" of a 4:3 movie on a 16:9 screen.
I can only recommend
Xee as an alternative viewer which always resizes its window to the images aspect ratio.
Works with multiple windows at the same time, no bars.
EDIT: If you resize the preview window to an aspect ratio with a greater height than the image and press "cmd+9" you have no bars left and right, but on top and bottom.
And maybe the following setting helps to open each file without letterbox (if available in Monterey):
Preview > Preferences > Images > Open each file in a separate window