Difference between amass and increase

Test No. [color=blue]incompl/advan-5 “Learning tips and instructions”, question 4

A useful way to your vocabulary is to read as much as possible.

(a) amass
(b) increase
© collect
(d) gather

Test No. [color=blue]incompl/advan-5 “Learning tips and instructions”, answer 4

A useful way to increase your vocabulary is to read as much as possible.

Correct answer: (b) increase
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Hi, I cannot distinct any differences between amass and increase in this context, can you help me, many thanks.

This means collect large anounts of something as in: They bought and sold several houses and over the years amassed a fortune.