The statement, "The more corrupt the government, the more numerous the laws," directly reflects the environment created by the pigs after they usurped control of the farm.
When the pigs initially rise to power, two dominant pigs rule the animals: Snowball and Napoleon. Once Snowball is driven off the farm by Napoleon, Napoleon becomes corrupted by his absolute authority. To maintain his authority and oppress the other animals, Napoleon must subvert the initial "Seven Commandments" of Animalism. The "Seven Commandments" were
1. Whatever goes upon two legs is an enemy.
2. Whatever goes upon four legs, or has wings, is a friend.
3. No animal shall wear clothes.
4. No animal shall sleep in a bed.
5. No animal shall drink alcohol.
6. No animal shall kill any other animal.
7. All animals are equal.
As the pigs become corrupted by their power and their innate intelligence further divides them from the other animals, the pigs assume a more "human" role on the farm. They live in the farmer's former residence, sleep in beds, and consume alcohol.
To resolve their behaviors with the rules of Animalism and prevent a second revolution, the pigs systematically change and adapt the rules of Animalism to their needs. Commandment number four now reads, "No animal shall sleep in a bed with sheets." Commandment number six reads, "No animal shall kill any other animal WITHOUT CAUSE." Ultimately, the final commandment reads, "All animals are equal BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS."
This manipulation of the Commandments illustrates that, as a government becomes more corrupt, the laws are more numerous (and in this case, more convoluted).
A counterargument could be made that Tacitus's statement is in direct opposition to Animal Farm, though. In Chapter 10, the pigs replace the Seven Commandments with a single rule: "All animals are equal, but some animals are more equal than others." At this point in the novel, the pigs are essentially behaving like people—they walk on two legs and Napoleon uses a whip. The pigs are at their most corrupt, but now only have one rule, which contradicts Tacitus's statement.