The forum software does try to fit the image into the available space as far as I know, but I would need to check that. What you describe could mean that the image is shown scaled to a smaller size in the original location, I assume you mean a website? It is always wise to check the actual image dimensions (in pixels) and adjust before uploading. That saves on storage space and potential display issues. There are other reasons that could lead to larger images than expected but it is difficult to comment further without knowing a bit more about the circumstances.
Edited to add: When you resize the PPRuNe window, the photos get resized automatically, so no real need to worry about this.