"they sometimes amounted to more than double"

Bank of England
Public DomainBank of England - Credit: Adrian Pingstone
Swift was a Tory writing during a Whig administration, and he is himself moved by those same party considerations he denounces.

As an example, the Bank of England was founded in 1694. The English government discovered that by borrowing money they could meet the financial requirements of a crisis such as a war without unduly increasing taxation. Then, supposedly, the debt would be paid off in peacetime. The War of the Spanish Succession was successfully funded in this way. Many governments have since dealt with crises by borrowing heavily. The debts are frequently left to accumulate, leading most recently to the sovereign debt crisis in Europe and the USA.

Generally speaking, liberals (such as the Whigs) concentrate on the good that borrowed money can do now, while conservatives (such as the Tories) focus on the troubles that might accrue later as the debts accumulate.

In Swift's time, the Whigs seemed content to allow the national debt to rise. The Tories denounced that practice. The Brobdingnagian King, in following the Tory party line, is a mouthpiece for Swift's own views.