no, in the long run animals are not getting smaller. In fact if anything the average size of animals are getting bigger.
There are two things you need to consider:
1. Spectacularly large animals were always the extreme outliers in size. The typical and the average size of animals were always much smaller. Humans might be small compare to some dinosaurs or elephants, but for anytime in the whole animal kingdom, or just land dwelling vertebrates, humans would count as a quite a large animal, larger than >99% of all animals whether by species or by numbers. The average size of the land dwelling vertebrate might be the size of a small dog or cat. It was probably never larger than that in entire history.
2. The different spectacularly large animals mostly didn't live at the same time. They lived millions to hundreds of millions years apart. So looking at a list of spectacularly large animal gives a completely wrong impression of how many large animals were in the echo system at once.
There are two things you need to consider:
1. Spectacularly large animals were always the extreme outliers in size. The typical and the average size of animals were always much smaller. Humans might be small compare to some dinosaurs or elephants, but for anytime in the whole animal kingdom, or just land dwelling vertebrates, humans would count as a quite a large animal, larger than >99% of all animals whether by species or by numbers. The average size of the land dwelling vertebrate might be the size of a small dog or cat. It was probably never larger than that in entire history.
2. The different spectacularly large animals mostly didn't live at the same time. They lived millions to hundreds of millions years apart. So looking at a list of spectacularly large animal gives a completely wrong impression of how many large animals were in the echo system at once.
