HiFly260 wrote:Just in gallery, they seem to become larger than their actual px size
This could happen for example if you are using the grid layout and only have a single item uploaded, the grid would become a single 1x1 grid, and if the image is very small, it might get scaled up to fit the grid. Unlike the popup, the layouts will shift images to suit the grid and not vice versa.
There is another factor you might need to take into consideration here, and that is screen pixel density. Your screen might for example be 3840 x 2160, but displaying "like" 1920 x 1080 (CSS resolution), to create more natural sizing instead of making everything incredibly small. This is often referred to as "retina screen", and it allows images and fonts to become more clear although still scaled down. However, images will still scale up to maximum pixel dimensions to match the CSS resolution, which means images may look larger than if for example opened in Photoshop or another local desktop app.
HiFly260 wrote:was just wondering why for "popup" there are no such size options ...
The popup is designed specifically to showcase a single image on screen, un-cropped, without scroll at best dimensions for the screen, without scaling pixels beyond original dimensions (causing blur). It doesn't really make much sense to have scaling options within the popup. At best, you would be scaling an image smaller than it's capable of displaying on the visitors screen, degrading it's viewing quality.