I have several monitors. My spreadsheets, browsers, file managers each cover 2 or 3 monitors.
When WinStep starts, all application windows shrink to single monitors. Why?

Ok, so you seem to be running Winstep Xtreme.

Please post before and after screenshots.

Are the 3 monitors set to "extend these displays" in Windows or are you using something like Nvidia Surround or AMD Eyefinity where Windows sees all 3 monitors as a single display?

Check the reserved screen areas for docks, shelves and drawers (i.e. prevent maximized windows, etc). Also, check the reserved screen areas for NextSTART (including those you can manually specify for each of your 3 monitors in the General tab of NextSTART Preferences).

You seem to be a Winstep Xtreme user since 2020, so I assume this is something recent or you would have complained many years ago. When did it start happening?

Finally, are you running Start 11? Start 11 seems to have a bug with the reserved screen areas.