I hate that we all automatically spring into nostalgia and some sort of "kids these days" mentality when it comes to these sorts of questions. I think the main reasons for this change are generally positive not a side-effect of us all getting thicker.
One factor to consider is that we've simply had more time to get better at making ads that serve their intended purposes. Advertisers perform a lot of cognitive science and psychological research to develop strategies and techniques for cutting through to consumers, and so you'd expect techniques for communicating to their audiences to change over time.
Regarding print ads: colour printing of newspapers is a relatively new thing. That allows brands to apply very consistently their marketing across multiple media which wasn't always possible before. If you're really extending your question back 20 years, you should also think about how much desktop publishing and design tools have improved in that time. In 1992 it wasn't feasible to produce many of the complex layering, texturing and compositing effects we now see. A bunch of newspaper compositing was still being done by hand (literally cut-and-paste) as recently as the late 90s.
It's also apparent to me (but not something I have evidence for) that magazines especially have become much more typographically pure in the current era; that's partly due to the proliferation of digital typography and partly due to the improvement of layout and proofing tools. It's also a fairly popular design aesthetic. With the actual editorial content tending to be more stark and grid-based, it stands to reason that the advertisements would tend to avoid that aesthetic in order to draw attention to itself.
It's also almost certainly related to the increase in exposure to advertising we all have now, which makes us far more cynical than we once were. "Talking head" ads directly espousing magical effects of a certain brand of clothing aren't generally going to be well received by a more savvy audience.