That may be because humans are large mammals. We can weigh 150 pounds or more. Most living mammals in contrast are rodents and bats, and most of them weigh a few ounces or so. We may not be considered large compared to elephants and rhinos or whales, but we are large compared to most other mammals.
During the age of dinosaurs, which lasted about 150 million years, no large mammal the size of humans or larger existed. Most of the mammals were living underground, inside burrows, and they came out at night to feed on insects. There were some medium sized mammals, such as the platypus in Australia. There were medium sized marsupials living in North America, they were about the size of a house cat.
Things were quite different after dinosaurs were wiped out 65 million years ago. mammals evolved to be much larger in their absence. A mere 30 million years after dinosaurs became extinct, Indricotherium, one of the largest land mammal that has ever lived, had evolved. The climate may have been a factor too, because Indricotherium evolved at the same time that the climate cooled, and Antarctica froze for the first time in its history. Since mammals generate their own body heat, and since large mammals have proportionally small surface area to volume ratios, large mammals can conserve heat better during a cold climate but they have trouble avoiding heat stroke in a hot climate. The climate during the age of dinosaurs was hot, and that may have kept big mammals from evolving. Indeed Indricotherium became extinct when the earth briefly warmed up again, about 23 million years ago, when Antarctica thawed briefly before re-freezing shortly afterwards. Nevertheless, dog-sized mammals evolved within 15-20 million years after the dinosaurs were gone, when the earth's climate was still as hot as the age of dinosaurs, so climate appears not to be the reason mammals failed to attain large size before the dinosaurs were wiped out.
In ecology, there is a term known as competitive exclusion, and it predicts that two species that live in the same place at the same time cannot coexist if they have similar needs. Every species has its own niche, and no two species that occupy the same niche can coexist in the same place at the same time. One of them will be eliminated by competition, driven to extinction, or both of them will evolve to have different niches so as to avoid competition. During the age of dinosaurs, all available niches for large land animals were occupied by dinosaurs and other reptiles. A new species of mammal that tries to evolve to occupy the same niche as an existing species of dinosaur would face stiff competition and will most likely be eliminated.
OTOH, after a mass extinction, there are many available niches that are suddenly open and unoccupied. In the absence of competition, new species that can evolve to fill these niches can easily survive. That is why we see an adaptive radiation, or a proliferation of new species after a mass extinction. After the dinosaurs were wiped out, the surviving mammals underwent adaptive radiation and they evolved into the variety of large mammals we see today. The ancestors of the living orders of mammals had evolved just 5-10 million years after the dinosaurs were gone. Giraffes, cows, hippos, horses, elephants, kangaroos, rhinos, lions, tigers and wolves all evolved after dinosaurs were wiped out. Their ancestors were tiny shrew-like mammals the survived the meteor strike that wiped out the dinosaurs.