I second the comment about small babies troubling people. It it not the obverse of comments about big babies (different issues of perceived criticism regarding overfeeding) Small, thinner babies in some cultures signify to some people lack of development and illness. This is of course, wrong, and children come in all sizes. But it's a pervasive myth that healthy children are 'over the 50th centile' and that you should do whatever's in your power (mix feed, wean early) to get them over this golden average.
I have a stunningly beautiful (no really, she is), slender two year old. She was 4 weeks early, was 5lb and just small. I'm small. Looking back over the photos she really chubbed up in the first four weeks, but I couldn't see that because I was wracked with nerves and worry and constantly told by Chinese people (I had her in Singapore, where some people like to overfeed their babies my paed said because they favour chubby babies) that she was too small, underfed, sick, etc etc.
I had to accept that some people didn't find her as attractive because she didn't have fat cheeks or any folds.
It coloured my breastfeeding relationship in the early days. I fed 18 hours out of 24 and the other 6 were spent weighing nappies and reading Kellymom. I was a wreck. It took about 4 months to feel less nervous.
I still breastfeed her at 23 months and she's never had formula. Weaned BLW at 7 months and she's a wonderful eater and she's still slim, but very active. But she has caught up imperceptibly and I no longer constantly compare her size with others. Although I did laugh when I saw a friend's baby yesterday - 5 months and she's almost as big as my daughter!
But even my mum, who breastfed me for 18 months, has made comments (more along the lines that I'm too attached and silly for co-sleeping until recently, and not letting her cry) and wasn't that supportive. But we have issues anyway.
You are unlikely to change MIL's opinion by talk of immunity, breastfed charts, WHO advice.
You're just going have to gloss over it/downplay it/lie/change the subject/turn the subject to other milestones.
Oh, and yes, it hurts when one grandparent seems more into one child over another, or that one grandparent is more supportive than another. My MIL is crazy about my daughter and can't praise me enough (just wait until I get on to the subject of my unconditional parenting, then she'll go nuts). My own mother hardly bothers to see her .
Good luck, and know that you are feeding her the best stuff and setting out a good basis for your future relationship.