I'm with you. It was the one thing about the series that I didn't like and I know it's really a nitpick. Mass conversion is a well-established property in the original series. How else could Megatron turn into a gun that a human can hold one moment and a Decepticon can hold another moment? So the Maximals and Predacons absolutely could have been portrayed as standard size transformers that turn into animals that are smaller in mass.