Star Trek

I don't know but I guess what ever he eats or drinks or eats gets dissolved. However now that you mention this how come you never see bathrooms or stalls in Star Trek?

Star Trek Toilets -

http://en.memory-alpha.org/wiki/Toilet

 Data would need some form of bio-mass refinery system in order to extract the required energy from any biological mass he might consume. Any remnants would need to be discarded just as it would in a carbon based lifeform.  Scientists are current experimenting with biologically based power sources for machines.

 Actually, I don't recall any specific instances of his eating anything.  He usually just feeds spot.

I don't remember I would need to see the finale episode.

If Data could refine biomass into a form that could be used by his systems and such a system could store excess energy within his body, then yes, he could get fat.  How fat would depend on how the system was programmed.

Or are we supposed to be answer this as if we were Data?

I could totalally picture a obess Data

From the TNG episode "Déjà Q"

Q: I've never eaten before. What do I ask for?

DATA: The choice of meal is determined by individual taste.

Q: What do you like?

DATA Although I do not require sustenance, I occasionally ingest semi-organic nutrient suspension in a silicon-based liquid medium.

Q: Is it good?

DATA: It would be more accurate to say it is good for me, as it lubricates my bio-functions.

This is a most disturbing question. And Data is just too innocent for.. that kind of things

I don't remember Data ever eating. He has drank before.