Hi Robert,
The reason for that is due to the event calendar plugin itself. Your plugin set specific size of the content depending on the size of its container. And since second, third, and other inactive tabs are hidden by default, then container’s dimension is zero. And thus your plugin serves the smallest sizing, please check this comparison
The imported CSS is for medium and small (second/third tab).

No CSS is imported for active tab content(standard CSS)

And here is the video recording of how your plugin scripts works https://www.dropbox.com/s/3bkmfd38ft6vab9/bandicam%202020-02-02%2003-12-33-760.mp4?dl=0. For every window resize, it recomputes the new size of the content, and then all other inactive tabs (even the first tab) became small again. You can confirm this by adding another content not related to event calendar plugin, like standard content, images, and so on, and their sizes aren’t changing compared to the content added by event plugin. For that, I recommend contacting the event plugin developer to see what fix they could add.
Thanks!