Drs look at "average" sized pregnant women and see danger! stamped all over her... it must be even more exaggerated for a woman they also see stamped as "unhealthy" just based on her size. :
Also, as much as it's worth anecdotally, my family history also refutes the MIL's argument.
Sister 1: severely overweight/obese, short (100% natural labours) (under 6 hours) with all three, her final labour lasting 2 hours total, baby born in an intact caul after 2 pushes. (sounds terrifying to me, but she swears up and down it wasn't that painful) She claims her pain was moderate, and this was flat-on-her-back hospital-style labour.
Sister 2- skinny/fit (she was a model and swam daily while expecting baby #1) Short (also natural) labours (baby #1- 5 hours, baby #2-induced, 2 hours total) but she describes them as excruciatingly painful. After baby #1, she desperately wanted an epidural for #2 but labour was way too quick. She refuses to have any more babies because she finds labour intensely painful.
Myself-definitely fluffy (cute term, BTW) Baby was stubbornly posterior. Labour was definitely manageable, then membranes ruptured and I needed the pool. The water took contractions from an 8/10 to a 4/10 until transition, which was rough but hip squeezes got me through it. Baby was pushed out in 'record time' according to midwife (first baby + posterior, I was sooo lucky to be pushing for less than an hour) and that night I was talking about having another.
I don't think weight alone has that much to do with it, fitness probably does but then again the key point is skinny doesn't automatically mean fit.