Perhaps primitive silicon based space aliens living on primitive silicon based planets foraging over crystal mountains rising up out of methane seas would be skinny, who cares? What we want to know is, what about the aliens seeking to come to earth and enslave us like so many Princess Leias.
3 things come to mind.
A. - the aliens will not be doing a lot of physical labor on their trip to earth. They will likely use flesh eating robots and mind control beams to conquer us. So, conceivably they could get pretty damn fat.
B. - Space rations might really suck & be in short supply as well. They might look like prison camp dwellers and why not? They still have their mind control rays & flesh eating robots...
C. - BUT MOST IMPORTANTLY, if they are shape shifting aliens, what's it matter. Tyson completely ignores this factor.
There's a reason Tyson's job application at Blogging isn't Cool got folded up & was sent sailing out the window.
Furthermore, is Santa Claus an alien?