system
Given the following post, respond as if you're commenting on Reddit:
human
What evolutionary pressures caused some dinosaurs to become so large?  

What evolutionary pressures caused some dinosaurs to become so large?   And why are those pressures not present today?
gpt
Short answer is, we really don't know. We can hypothesize, and we can speculate, but that's about it.

It should be noted that pressures for gigantism are still present today. See, for example, elephants, rhinos, and hippos. There are advantages to getting big; less chance of predation, easier to maintain internal body temperature, and more efficient food intake (for example, compare yourself to a mouse. You require more food total, but a mouse needs to eat its own bodyweight in food very frequently--something which takes you several months usually).

The question then becomes, how did some dinosaurs get so freakin' gigantic? Most big ones got about the size of an elephant or so, but some got even larger. 

Now, there's a couple points to discuss before moving farther. 1) anatomy. We do have (very) good evidence that many--if not most and possibly all--dinosaurs had extensive air sacs in their bodies attached to the respiratory system. Birds have the same thing (makes sense, since they are a subset of dinosaurs) and crocs have something similar though less extensive.  2.) size vs. mass. When we say dinosaurs got "large", what do we mean? Do we mean they were physically big, or that they were immensely heavy? They're not necessarily the same thing. Because of those airsacs, it is actually tricky determining how much those big dinos weighed, because we're not sure how much of their volume was made of of those airsacs.

But one thing IS unmistakable--some dinosaurs (e.g. some sauropods) got very VERY big. But how did they do this physically? One idea is that it is actually related to those airsacs. You see, in birds, those airsacs actually affect how the lungs work. In you, your lungs work like a bellows--air goes in, air goes out. In birds, it is a bit more complicated. Air flows into the airsacs, then *through* the lungs (in one direction) to another set of airsacs, and then is expelled. This means air passing through the lungs interacts with the capillaries in a counter-current mechanism. Bird lungs are actually much more efficient at extracting oxygen from the air than our bag-like lungs are. 

So, *maybe* sauropods had a similar lung setup as birds. There's actually pretty good reasons to suspect this, including evidence of airsacs in the vertebrae. If they did, this may explain why sauropods managed to get so much larger than elephants, and also maybe how they managed to get enough air to breath with those amazingly long necks.

**tl;dr:** the pressures are still there, and dinosaurs may have gotten larger than animals today because of more efficient bird-like lungs affecting the constraints on maximum size

**edit**: there's another aspect to this question: growth. There's two general ways to get really big: live a long time and keep growing, or grow really, really fast. Some large animals, like crocodiles, get really big by growing slow through long lives. Others, like elephants and whales, reach adult size within a few years. Bone histology of dinosaur fossils indicates that they grew very, *very* fast. The largest *T. rex* know, "Sue" on display in Chicago, was only about 30 years old when it died, and it was an old individual.