It is not the file size that determines how big a photo will appear on these pages. It is the height and width that the picture was saved with. In any photo or drawing program there is a canvas size in pixels and that is the default size that many BBs will use to show that image. So one of those 20 or more MegaPixel cameras will produce a photo that is around 5,000 (+/-) pixels wide and tall. My 21" desk top monitor has a resolution of 1680 x 1050 pixels. That means that only about 1/10th of a 20 MegaPixel image can be seen at one time. To see the rest I would have to pan and scroll that image.
The file format that the BBs usually want is JPEG which is a compressed file. A relatively small file can often hold those compressed images.
Around ten years ago the BBs did not process the images. In fact the BBs of that time mostly did not even host the images but instead relied on links to other image hosting sites. And very large images were often used leading to posts that were difficult to even read because the text defaulted to the image width and you had to pan left and right to read each line of text.
Then, due to lower prices for ever larger hard drives, the BBs started to host the images. And when they uploaded them, they automatically reduced the image width to 1000 pixels or less. This means that they would fit on most computer monitors without panning.
The new software for this BB seems to have come from a new company and I suspect that the automatic image reduction function has not been included. That means we are back to ten years ago where every image that is uploaded is displayed in it's original, native size in pixels.
My advise is the same that it was ten years ago. Before uploading an image, open it in Microsoft Paint or some other image editing program and reducing the canvas size to 1000 pixels or less. An 800 pixel width is a good choice. If you go with the default settings, Paint and most of the other programs will automatically reduce the vertical size in proportion to the horizontal width you choose and the aspect ratio of the image will be preserved. Then you can save with a new name, like XXXXX800Wide.JPG and use that new, smaller image for your uploads. That is what I do, just to save storage space on the computer that hosts the BB.
Edit, 6/22/22; "MPEG" changed to "JPEG" with my apology.