We have an obesity crisis it's a good thing food portions are shrinking some IMO.
I think the impact is just the opposite. It's easier to eat the whole container in one sitting because it is smaller and looks like a single portion size.
I used to eat a full bowl of cereal each morning. Then I started using a measuring cup. Then I used a scale. There is a big difference in portion size and calories. I was surprised at the difference between the measuring cup and the scale. The scale is more accurate and I found the measuring cup understated the calories by a good 10-20%.
There are a lot of factors regarding obesity. I also don't want to highjack this thread to talk about obesity. I suspect a significant factor in obesity is our mental propensity towards value...meaning you get a lot for the money. You see this in restaurants where you get a big plate of food for $7.99. It's way more than you need, but you see the value. We, in the US, like value. We see this in the food manufacturers not wanting to raise prices so they shrink the amount in the package, not necessarily the package size. So your brain thinks same amount for same price. Hence, in the news you hear food prices going up up up. But in the store, you see the same comfortable package for the same comfortable price. It's only when you do the math do you see the price per unit has really gone up 20+%.
I have switched to a lot of fresh veggies and fruits. So I really see the price increases as it's really easy to see. What used to be 3 oranges for $1 is now $1 each. I bought a bunch of fresh fruit in Oregon and didn't pay attention to the units on the pricing. It was different from my local store. So I thought $3.99 was a good price for a melon. It was $3.99 a lb. Ouch.
