|This ultrasaurus would certainly be better off if it was smaller!
Illustration from "The Depot".
Big animals are often clumsy and slow-moving. It is difficult for them to even lay down for long periods of time. They are able to outrun predators, but can only go short distances before getting exhausted. Their metabolic rates are higher so they need more energy than smaller animals. As they feed, they cover a relatively small area in a day, so the little animals get all the succulent vegetation first. Therefore, the giants cannot be choosy about what they eat. They need more food mass and water to survive. Population turnover rates are slower because an individual has very few and small litters per lifetime, and longer gestation periods than smaller animals. As a result, population size and density is lower, evolution and adaptation is slower, genetic variation upon which natural selection can operate is lower, the continuation of the species is dependent on longevity of individuals rather than population size, and the population is more susceptible to disease and extinction because of the lower birth rates. Maturity is reached at a much later time and, therefore, babies need longer periods of parental care. Their habitat specialization is lower, they cannot bury themselves to escape cold weather, and they cannot fly. Finally, their circulatory systems must be very efficient to provide energy to every inch of their massive bodies.
Plants are limited by size because they need thick soil to grow in and need large supplies of water. If the tree is too large for nutrients to be drawn up the trunk from the soil, the tree will not survive. Large trees can be susceptible to breaking in strong winds whereas the smaller ones remain sheltered.
The evidence seems to show that the negatives outweigh the positives, which is likely why there are so few large creatures relative to small ones.