From what I have seen I do also believe she doesn't have a healthy diet. She claims to be gluten intolerant (my husband is also) and yet as you say there is a lot of beige food for someone who is allergic to gluten. I have also also noticed even when they go away and can order truly yummy (good) food from a menu she often will have a pizza or something else gluteney.
After watching the good food Josie and Charlie prepare and eat it puts her and Alis diet to shame. Maybe she is very used to fast food from the Ibiza days.
As for her size maybe she is just one of these people who can eat high carb food and not gain weight, she doesn't look sickly or anything but I do think in the post you attach that she is showing off a bit, why post that???
The takeaway Sunday roasts are a bit surprising, why not try and cook it yourself? Make a video about it, like Josie does, not that I am a big Josie fan, she annoys me too but at least her content is varied. I do honestly think Lydia is too lazy to do it.
And trying not to be really nasty
BUT this could have something to do with her past, if she hasn't enjoyed Sundays spent preparing the roast meal, messing up the kitchen, having to do a big clean up and enjoying it all, maybe she does just see it as too much work. This is not her idea of an enjoyable Sunday. I better stop now I am going off pist, this woman amazes me and I could analyse her all day.