Food reporting is often framed like this, as if the smaller size was a bad thing, or the price / gram, pound or ounce is the relevant criteria. Esp in the case of candy, you would think smaller sizes would satisfy the other framing for food items, esp in America, that over-sized food leads to obesity. It is a lose-lose framing, with nutritional value completely ignored. Instead, the reporting is always how people are being ripped off or making an unhealthy choice.

If the drop in weight was accompanied by a drop in price I think the health angle would stick better.

Your framing makes it seem like these companies decided to reduce the size of their products for the health of their customers. We all know they're doing it to save on costs and increase profits.

Why would news outlets go the extra mile and re-frame it as "Companies are reducing the size of unhealthy products while maintaining the same price and here's why that's a good thing..." unless they were in the pocket of these companies? That would be editorializing to me.

Is obesity more or less prevalent now versus 1991?

the common denominator is that most "news" is pure entertainment and outrage porn sells. there is no further substance

The bad thing isn't the smaller size, it's the deception that goes along with it. Sometimes companies introduce a smaller size as a smaller size, and nobody cares when that happens. Small size candy bars are common and everybody's fine with it. The uproar is when they shrink an existing product while hiding the size change as much as they can.