Web pages randomly expand to 400% view

Hello landscrapeartichoke,

It sounds like you may be inadvertently zooming on a page. For help with gestures, shortcuts and keys that zoom on your Mac check out the steps below found in the article here: How to zoom in or out on Mac

"Use scroll gesture with modifier keys to zoom

After selecting ”Use scroll gesture with modifier keys to zoom,” you can press and hold the Control key while scrolling with your mouse or trackpad to zoom. Use the ”Zoom style” menu to choose from three zoom styles: full screen, split screen, or picture-in-picture.

Full screen zoom

Split screen zoom

Picture-in-picture zoom"

There are additional examples in the article provided. The page here also has helpful information: Zoom in on webpages in Safari on Mac as well as the page here: Change the font size or zoom level of web pages in Safari

Best.