The Elephant is the heaviest land animal that has existed in recent times. A heavier animal could not exist because its bones and muscles could not support its weight. The tallest land animal is the Giraffe at 5.8 metres. A taller animal than the Giraffe cannot exist because a heart cannot be powerful enough to pump blood to a head higher than this and the animals blood pressure would be too high to live. The heaviest flying bird is the Great Bustard weighing 20 Kgs and with a wing span of 2.5 metres. A bird heavier than this could not support wings powerful enough to fly, therefore Pterodactyls would not have been able to fly.
The Expansion of Space.
Having established that massive dinosaurs could not physically have existed, the alternative is that the fossilised dinosaur bones have increased in size with the passage of time. In the beginning all matter was packed together as a singularity and space did not exist. The "Big Bang" happened, space began to expand and has been expanding ever since. Particles of matter were spread throughout the universe, with space in between them. The amount of matter remained constant but as space expanded it became less dense. Matter was not spread evenly, but gathered together in clumps. Clumps of protons, neutrons and electrons are held in association by electromagnetic and nuclear forces. Atoms, stars and galaxies are held in association by gravitational forces. Between all particles of matter however there still remains space. Ninety nine percent of the volume of an atom is space.
Since all space is the same, it is not only the space between galaxies that is expanding, but the whole of space, everywhere, between everything. This includes the space between atoms and the space between individual protons, neutrons and electrons which make up atoms. This means that the volume of everything increases with time so everything becomes "bigger". Most of the time we are unaware of this as the whole of the universe including our selves expands at the same rate, very slowly by our time scale. Dinosaurs were in fact "normal sized" land animals but their fossilised bones have become bigger because of their increase in volume due to the expansion of space over millions of years.
While matter can move through space if a force is applied to it (eg rockets or comets) the nuclear, electromagnetic and gravitational forces holding atoms or galaxies together remain constant. They do not increase or decrease. They exert no force on space its self so cannot stop, resist or prevent the expansion of space. If they could then at the Big Bang gravity would have held all matter together and space would have expanded away from it creating an empty universe with a singularity at its centre. Gravity does not slow the expansion of space. As space expands forces do still maintain a relationship between particles, atoms, or galaxies but the distances between everything increases, so the volume of everything increases. In order to keep the same distances, nuclear, electromagnetic and gravitational forces would have to increase, which they do not.
The doppler effect gives an indication of the expansion of space on a universal scale but there is no way of measuring expansion on a subatomic scale. If however as an example, we guess that an atom takes 100 million years to double its volume due to the expansion of space, the double volume atom is still a very tiny thing. A dinosaur bone fossil however, which is made of atoms, buried at the bottom of a lake for the same 100 million years will also double its volume and therefore suggest a dinosaur twice the size that it actually was.