Jonnyfan, I know some people don't really see other's size, and it could be your friend mentally thinks of you as 'the large friend' and just hasn't adjusted her mindset regarding your new size. It's hard to realise that when so many of us are very quick to notice the difference in other's sizes!
OP - you could be being over sensitive, both those instances could be seen in a more positive light - the pasta dinner doesn't have to be unhealthy, on most diets, carbs are allowed so a low fat sauce on wholemeal pasta could be a kind offer of a 'diet' meal. Most people would assume 'on a diet' means 'low calorie' so as long as a meal is low cal it would be ok.
the cake is harder to explain, unless your friend didn't think about you being on a diet - if I cut two slices of cake for myself and a guest, I'd always offer the guest the larger slice - in the same way I'd always offer guests the 'best' of whatever was on offer in my house, it's standard 'good host' behaviour, she probably did it automatically without thinking.
I know you are probably focussed on your diet and retraining yourself to be more restrained, but you have to remember that other people won't think about it, and won't automatically know what sort of diet you are doing. Just practice smiling and saying "no, but thank you for offering" - don't bang on about your diet, they will eventually learn you don't eat as much as you did.