Vladimir Harkonnen, Baron of House Harkonnen during the period when Paul Atreides became known as Muad'dib, was a human that looked and acted remarkably similar to Jabba the Hutt, a creature from another galaxy, far far away.
In fact, the Baron and Jabba were both fat tubs of lard that needed mechanical help to be reasonably mobile, both had wrinkled, pus-encrusted skin, were sex-hungry (the baron preferred men, however) and ate delicacies that made even Klingon Gah look appetizing. They also both were essentially crime lords and both had control of desert planets (Dune & Tatooine) and each one of those planets had deadly creatures that lived in the sand (Sandworm & Sarlacc). Jabba and the Baron loved to watch people get eaten alive by creatures or die in gladiator-style games (Rancor & Feyd-Rautha Harkonnen) and were scheming creatures that thought they were incredibly intelligent. I am not sure the baron was able to resist Jedi Mind Tricks however.
Metafist: If you are able to read the words above carefully, which have not been edited since the node was first written, you will see that it does not say that Vladimir Harkonnen is Jabba the Hutt, nor does it say Vladimir is a crime lord. The writeup says they are remarkably similar in various ways, and that Vladimir Harkonnen is essentially a crime lord. See stain. Finally, it is not important at all that Vladimir was Baron of House Harkonnen long before the period of Muad'Dib. What's important is that he was Baron during the period when Paul Atreides became known as Muad'Dib.
Moreover, while Dune: House Atreides may be entertaining litterature, it is in my opinion not canon. It is a valiant attempt to stay consistent with the truths of the Dune universe, but shouldn't be considered as part of the real story of Dune. So we don't really know why Vladimir Harkonnen is so fat.