In lang=en terms the difference between curtail and increase
is that
curtail is to shorten or abridge the duration of something; to truncate while
increase is to make (a quantity) larger.
As verbs the difference between curtail and increase
is that
curtail is (obsolete) to cut short the tail of an animal while
increase is (of a quantity) to become larger.
As nouns the difference between curtail and increase
is that
curtail is (architecture) a scroll termination, as of a step, etc while
increase is an amount by which a quantity is increased.