Safari suddenly displaying preview page of most webpages

That is indeed an annoying effect, too.

Have you tried various Safari routines..?

(Any chance you may have turned on 'Safari Developers' menu in Browser?)

Clear your browsing history in Safari on Mac - Apple Support

These could have a desired effect ~ to return to Safari to defaults.

• Safari User Guide for Mac - [Table of Contents] - Apple Support

//support.apple.com/guide/safari/toc

There also are 'reading list' features; never used those custom settings.

A larger volume of Safari support articles are linked within User Guide.

Keyboard shortcuts and gestures in Safari on Mac - Apple Support

(exit full screen view in zoom?)

• How to zoom in or out on Mac - Apple Support

//support.apple.com/en-us/HT210978

• Zoom in on webpages in Safari on Mac - Apple Support

• How to Set a Default Zoom Level in Safari for Mac - How-To Geek

https://www.howtogeek.com/658782/how-to-set-a-default-zoom-level-in-safari-for-mac/

By accident, had same effect in iPhone iOS, but that never stuck there.

Could be useful to archive or print several long messages in iOS.

The Safari 'Reading List' article has some interesting aspects; those may

not apply to this instance however, but to re-set the browser could help.

Set up iCloud features on Mac - Apple Support

Maybe something was set in iCloud preferences, those effect across

several devices; IF you see that happening on other Safari versions.

..Sorry to not have any exact cure that may pin-point a cause..

Three browsers in my Mac mini i7 (four macOS) dual drive; two partitions each.

Brave, Safari, Firefox. Those seem to be able to work together, except Brave is

very touchy given default security is high; but that also works in older macOS.

My belief that your system with large long preview of articles, is a matter of settings.

[The same effect occurs in iOS iPhone/Safari, but by accident. It hasn't stuck yet!]

..Could be Zoom settings in Safari, defaults, etc. Something fairly easy too..

+ iCloud sharing of network/browser settings, may be worth looking into.