All images on my site are shown with thumbnail size. It does not matter if I create galleria, carousel or unique image, in all the same thing happens, the lightbox always shows a miniature.
I have reviewed everything and I can not find how to solve it.
It’s been so long since you haved this trouble, but for those who have this issue i post this answer (even if you still needed 🙂 ). When you select some image on the WP modal, view in the right side bar and scroll down, there some options to select the image size, just select full size. It’s probably as default it’s selected the medium or small option.