If your post list thumbnail on mobile is too small you can go to Customize > Blog/Archive > Thumbnail > Post list: Thumbnail width to adjust it.
This applies to Archive post list like Category page, Author page.. In case you need to adjust post list thumbnail width on mobile, you go to Customize > Homepage Builder > the section > Elements tab > scroll to Thumbnail area.
There’s another solution, is to set post list layout to be stack on mobile, ie. Thumbnail above and text below.
For archive pages, you go to Customize > Blog/Archive > Post Item > [LIST] Mobile layout