Yeah, this isn't anything new.
One food product to look at is chocolate:
You see how prices spiked pretty quickly after the '70s.
Also note the changes in Hershey Bar prices/sizes here:
The Food Timeline--historic food prices
Since the era of packaging, food marketers have had the challenge of balancing prices with portions. The thing to realize is that companies such as Hershey have the external pressure of global trade. Cocoa and sugar prices are beyond their control, so they manage it with things they can control: product size and price.
The same goes for potato chips and tuna, and anything else, really. When people are used to paying a certain price for some things (such as candy bars, bags of potato chips, and cans of tuna) it's really difficult to increase the price without some kind of backlash, especially when you have competitors. So what to do to remain profitable?
Reducing portions is easier to get away with than increasing prices.