thumbnails in 'page & layer gallery' different sizes

[leonard55]

My thumbnails in 'page & layer gallery' are different sizes. See picture.
I thought it was because I have different length pages, but there appears to be no rhyme or reason as some longer pages have wide thumbnails,
some long pages have narrower thumbnails, and
others the same length have different width thumbnails .

Looks like maybe the longer the page is, the narrower the width of the thumbnail, but that is not quite right.

There must be s simple explanation. What does this indicate?
Thanks, Leonard Slates

[Acorn]

Xara's centrally fitting your thumbs to a fixed height, scaling accordingly.
When a thumb fits width wise, only then will it be not as tall as the fixed height.

Acorn