The smaller the bird the easier it’s for them to fly, cause they are lighter

Yes and no. Take the humming bird, for an example. They are very small, and light, but the way they fly, flapping their wings back and forth extremely fast, and the metabolism running to power that, means they have to feed constantly, where as some of the larger birds, much heavier, soar on thermals, eat when they can, and don't exert much energy while in flight.