Photobucket used to have preference settings as to how to handle larger image files, like those over a meg in size, for example. Maybe they still do? It's not about pixel size, but file size. It appears that when an image exceeds a specific file size (used to be a meg, I believe), auto compression / reduction on upload kicks in.
Personally I reduce the dpi and resize the image size to create a "screen resolution version" when I post to Photobucket. Then Photobucket doesn't mess with my images. All that extra resolution, while quite necessary and desirable when printing an image, is not necessary when viewing an image from the web on consumer grade monitor. A small side benefit is image thieves and Google Images only get the low resolution photo.