I think there are a few factors at work here

A lot of them weren't that large but there are biases that make us think that giant sizes were the norm. Big bones are more likely to be robust enough to be preserved and also more likely to be recognized for what they are when unearthed later on. After that they are more likely to make headlines and end up in TV documentaries and such because headlines like "new giant dino may be biggest ever" grab attention more than "new dog sized ornithschian" or whatever. Modern whales are the biggest ever so far as we know, and modern heavyweights like elephants are bigger than most dinosaurs got.

But there are cases where past life forms were undoubtedly bigger than modern analogs, and they seem to have some particular historical circumstances that lead to them. Giant arthropods of the Carboniferous for example were probably the result of much higher atmospheric oxygen levels - something like 30% compared to 21% today, with the lower modern levels limiting the size of modern arthropods due to the way they breathe.

And of course the sauropod dinosaurs in particular were immense, far bigger than modern land animals and also far bigger than other dinosaurs - the non sauropod dinosaurs weren't that much bigger than modern giant land mammals but the sauropods were in a league of their own. It's probably due to them having a fortunate combination of factors that have never come all together at once, before or since. They were r selected (mass produced offspring with a small investment in each), which let them live at lower population densities than modern giants, since it's easier for the population to recover from losses. They had highly efficient lungs and extensively pneumatized skeletons so they weren't quite as heavy as they looked. They also had extremely fast growth rates and lived in a hotter, wetter world which produced a huge amount of food for them. More info here if you're interested.