As another European I agree. When we eat in a restaurant with my family its almost always the case that for some people the portions will be too big and for others too small. What we sometimes do is we just pick a dish for each person and each of us eats about 1/n'th of each dish, with the people who are more hungry eating more and the others less. I'm not sure if this would be considered bad manners (not that we'd change it, we're tasting n dishes instead of one and having more fun comparing what we like and don't like, especially if the restaurant is good).

I'm pretty sure that taking food with you in a bag is unfortunately considered bad manners :( Asking for a second plate and silver maybe a less so but still.