Dickens seems to be guilty of a bit of prejudice:
"The
third party, which had ascended from the valley on the Italian side
of the Pass, and had arrived first, were four in number: a
plethoric, hungry, and silent German tutor in spectacles, on a tour
with three young men, his pupils, all plethoric, hungry, and
silent, and all in spectacles."

A "plethora" can mean an abundance, an excess of something.
So a plethoric person is stout, looks overweight, and possibly hypertensive. I doubt that spectacles are more common in Germany than in England.

Webster's 1913 Dictionary

Definition: \Ple*thor"ic\, a. [Gr. ?; cf. F. pl['e]thorique.]
Haeving a full habit of body; characterized by plethora or
excess of blood; as, a plethoric constitution; -- used also
metaphorically. ``Plethoric phrases.'' --Sydney Smith.
``Plethoric fullness of thought.'' --De Quincey.