Are portions so small in restaurants in hope that you'll buy another meal ?

Often they are too large.. When they aren't, they are small. I doubt they design it for you needing more food, it's likely just food is expensive, labour is expensive.. and society tends to associate too, more expensive is good.

Both probably. The restaurant thing is often intentional, but on average our portion sizes at home have massively increased over the past 20-30 years.
In the US.

I'll look in my crystal ball for an answer but first, how overweight are you?

Restaurant portions are with the idea of a few courses meal. At least 3 courses.