To me "fattest hit" means the thickest, most lung-busting hit of vapor delivered; which seems to be how most people are rating their vapes. Which obviously varies based on a number of things. People need to stop over thinking this thread; despite all the information out there sometimes its reassuring to hear what other people have to say about their vaporizers vapor production.
Obviously someone isn't going to rate one vaporizer low because they put a pinch of shitty mids it and didn't get much vapor then rate another one high because they loaded a big bowl of top shelf headies topped with keif and wax. People have enough experience with their vaporizers to say which produces the most vapor for THEM. Albeit is hard to compare vapor production in forced air and bag style vaporizers to whips and so on, just take everything with a grain of salt. But you can gain some idea about vapes by comparing similar vaporizers in peoples lists. Not all things must be scientific. You're a dumbass if you believe the information in this thread is objective and has any real meaning outside the opinions expressed by the people that posted. Personally I like seeing how people compared their logs and portables to one another.
Its a fun thread, post on!
Edit: Didn't realize how old that post was, oh well...