Responsive pop-ups

Hi everyone,
I am currently developing an app with the mobile first approach- all layout in 320px width, with groups moving up when the page width expands. This has worked well so far to tackle responsiveness until now. I tried creating a pop-up to handle a certain operation and went to the responsive screen-only to find that the only options available are for the pop-up to shrink in size or remain static i.e. you can only set a minimum width or a fixed width and the pop-up actually does not scale in width as the page expands. Is this the expected behaviour? Any guidance on how others are handling responsiveness in pop-ups will be helpful.