No, it's just a random sentence. But it's not hard to interpret it sexistically. Unless we also have the reverse sentence (En tjock man älskar en smal kvinna) it probably should not be in the course. I'll make a note to follow up on that later.
So “små” and “smal” both come from the same origin but mean slightly different things? Is the latter a German loanword?
No, they're not actually related - they're from different Proto-Germanic roots, much too old to be loanwords.
I wouldn't say "slightly different" either - små is the plural form of "little", and smal means "narrow". But I suppose that's a matter of definition. :)
In the English translation both "fat" and "thick" are accepted. For me (Brit Eng Native) "a thick person" only means a stupid person and means nothing about their size. Sooooo... if (a)"tjock" referring to a person in Swedish is only ever about physical size and not their intellect, and (b) a "thick person" in general English (not just mine) is only about intellect and not physical size, then maybe it should be altered.
"thick" as in "thickset" is common throughout large parts of the English-speaking world, so I think it should still be accepted. It would be preferential if the system hide it from users except for purposes of accepting translations, though.
A thick woman loves a thin man:
Jack Spratt could eat no fat;
His wife could eat no lean.
And so between them both, you see,
They licked the platter clean.
At least this thick woman and this thin man had a working relationship. :-)