In the past, Earth's gravity was less? Why giant dinosaurs were flying?

It is very curious how the ancients were flying species which weighed 160 kg and that could fly safely, and now no bird that weighs less than 15 kg can not fly because of gravity. For example, weighing only 11 kg, eagles in Central Asia have difficulty in rising from Earth and can fly. And then, how come the time of dinosaurs there were species that weighed over 100 pounds and could fly? In conclusion, it appears that in the past was much less than Earth gravity!
A scientist named Tom Holden, examining animals on Earth was in the past, concluded that all gravitational conditions of the past were different to those we know today, and now that dinosaurs did 65 million could withstand the new conditions on Earth.