WE do not need to care about "rather than", and "instead of" for this problem.
- "increase in something" is idiom
the second thing is the use of doing
when there is a noun, we do not use "doing" to refer to a general action but use the noun. when there is no noun (there is no "learnation"), we have to use "doing" to refer to a general action. but remember, "doing" is often used to refer to an action of a specific noun in the sentence. for example
I learn english by speaking it all day.
in this sentence, "speaking" refers to "I".
there is justification for the use "increasing" in this sentence because there is noun "increase". This is gmat rule, which is shown in many gmatprep questions. we have to infer this rule by studying gmatprep questions because no grammar books explain this rule.
If anyone in this gmat forum is in England,Britain, pls, email to me, (firstname.lastname@example.org) . I have some questions and need your advise. Thank a lot.