Zoom slider. Bottom right corner of the GUI. You can set separate zoom factors for Preview, BooK View and Code View. Perhaps you've inadvertently changed the default zoom factor for Preview? I've done it accidentally before with an accidental keystroke and/or mousewheel combination.
Click somewhere in Preview and hit ctrl+0 to reset the zoom to default. You can do that for Book View and Code View as well if they've been changed. Just make sure you've clicked on the widget whose zoom factor you want to alter.
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