The dirty little secret of Lost World stories it that, for the most part, modern predators are likely more efficient and dangerous. However, some older mammals tended to be bigger, which is cool.
Then there's the "Jurassic Park" fallacy that many overly romantic paleontologists ascribe to... y'know, the idea that velociraptors were twice as big as they really were, faster than cheetahs, smarter than chimps, etc.
The real secret, and it's actually neither secret nor dirty, is that all faunal assemblages, i.e., the collection of animals that are found together in nature in a given place and time, are well adapted to the situation in which they find themselves. There's an intricate relationship between prey, predators and environment, and all three are highly adapted to each other. Animals go extinct because their environment changes, their foodsource changes, or other changes cause their specializations to be the
wrong ones for the new environments in which they find themselves. More generalist creatures then pick up the slack and evolution causes them to in turn become more specialized.
Evolution isn't a constant
progress---on to better and more efficient things. Rather, it's just constant
change---on to something new because a few inputs in a highly complex system change. I think it's a mistake to assume that modern predators are "better" than extinct ones just because the extinct ones have the bad-luck to have gone extinct.
