Whilst the single (very flimsy) voucher did have it all and the ATB boarding pass was the standard 'DL' envelope size, two things have changed: Money + Legal requirements.
I'm just back from a long trip and most of the travel agent's 17 pages were taken up with legal stuff. Each item had standard reminders about what I was supposed to do [so that they would be able to prove that they had done all they could to tell me what to do] and it was formatted with very wide margins wasting space. I just put the PDF on all my electronic devices and read it from there.
Once LCCs started getting pax to print their own boarding card - they knew that folks would print on A4, so they started selling advertising. I just loaded the actual boarding pass (still the same shape) and put that on my phone. I carried minimal paper.
All carriers want to get away from paper and much progress has been made but the inertia in the system is enormous. There are a lot of airports to convert to paperless and a lot of (usually older) pax who still like a piece of paper. Over the next 20 years we will see the generations change and each airport refurbishment will move it closer to electronic.
However, the printing of legal excuses and @rse covering will continue ...
