Translation:The girl is big because she eats a lot.
I understand "grasa" to mean "fat" and "granda" to mean "big, large". I think this sentence implies "She's grown big and strong" (to use the common English phrase) because she has had plenty to eat. But I would look to a more knowledgeable Esperanto- speaker to confirm my interpretation.
Not an expert, but according to: http://en.lernu.net/index.php and http://esperanto-panorama.net/vortaro/eoen.htm -- granda: big, tall, great -- grasa: fat, fatty, greasy. I believe the confusion here is: 1) Girls aren''t supposed to be "big and strong". (Why, I don't know. I wish I was.) 2) At least in U.S., we have an overabundance of unhealthy foods and eating "a lot" is associated with being fat, not fit. As a teen, I remember a friend's grandmother used to tell us "Eat, be strong and big, like bull!"
Good question. This is the only information I found on commas (or punctuation) with Esperanto: http://en.lernu.net/lernado/gramatiko/demandoj/interpunkcioj.php
Maybe the context for this sentence is just the opposite: a place where food is not quite available so most children don't even get to their average weight, but in this case we have found a happy exception: "The girl is big (therefore healthy and strong) because she eats a lot". Or maybe twin siblings born underweight and one is doing fine with the eating and growing accordingly but the other is not.... Just thinking out of the box! :-)
Neither is very common in my experience. For this, I wrote "because."
To me "as" implies that these are simultaneous rather than cause and effect.
Also, even though neither is common, which one is used more depends on the people around you. I almost never hear "as" used in this way. "For" on the other hand, I do sometimes hear.
Lastly, if your answer was marked wrong an you feel it should be accepted, report it. Then the mods can review it and if they feel it is common enough, they will add it.
Calling it absurd in the comments is much less effective.
It's a both-ways sort of thing. Many countries dance around facts so as not to offend, and many are more blunt, but I'll bet you'll find examples in almost any country where they say things that other countries find offensive like "Why would you ever say that?!"
Some cultures easily talk about religion, while others find that appallingly offensive unless you know someone incredibly well. Some cultures easily talk about money, while others go out of their way to not acknowledge economic inequalities of any sort. Some cultures easily talk about health problems in public, while others go "No, sitting at a bus stop next to complete strangers is not the time to be discussing my recent bout with diarrhea!"
It's not that one country is dancing around all the facts in plain sight, it's that each culture has a different set of facts that they consider it rude to discuss.
All of which is making me wonder which cultures could be brought in to solve certain problems. Like, maybe if Hungarians are quick to point out when you don't look like you've been eating well (I don't know if this is a characteristic of Hungarians, I just needed an example), then a Hungarian friend or relative might be better able to work with a person who's got an eating disorder, when other friends and family just try to pretend it doesn't exist.
I know a lot of people in America just try to ignore it when a kid is out of control and the parent isn't doing anything to corral them, but I've read anecdotes about French people just coming up to a kid and telling them to behave like why wouldn't a complete stranger just do that? Because the expectations for public behavior are different.