change the resolution of preview app

I wanna ask you about Preview app for Mac.

As you see the red arrow in the picture, there is some space between a color page and a BW page on it. I wanna adjust the size so that all of the pages are the same size.

the resolution of a color page is 21 × 27.18 cm

the resolution of BW page is 16.83 × 23.68 cm

I wanna convert the resolution color page's to BW page's, but after doing it, the display is smashed.

How can I convert the resolution of a color page's to BW page's as the page is not smashed? Even if the page has white margin is ok.