I've always been very naturally thin, and I have had instances in the past where I have complained that something I was wearing made me look bad and it has upset a friend of mine who is a little bigger. The issue for me was because I myself didn't see my friend as being overweight or unattractive, it didn't occur to me that me putting myself down would effect them. I can see the same thing with Jade being around Mikkayla. Jade doesn't seem to think there is a problem with the way Mikkayla looks at all (it's Tahan, Tully and Tim that have issues with it), so I don't think she realised that making comments like that around people like Mikkalya was an issue. I've been there, and it's not usually until your friend actually says something that you realise the implications. You were only worried about your own insecurities because to you there is nothing wrong with the way your friend looks.
And let's be honest here. That dress did make Jake look pregnant. Tully told her herself.