Chaucer and Shakespeare made efforts to increase English word stock. How?
I'm not sure either poet actually made a conscious effort to increase the word stock of English. But they both did.
The words that Shakespeare invented, words which he made up totally afresh for the first time, include assassination, sea-change, manager, director... and about 10,000 more. He is one of the most significant expanders of English word-stock.
It is certainly true that in Shakespeare's time the language was expanding anyway due to words coming in from foreign language and the influence of the printing press. But in the year 1602, 250 new words and meanings arrive into the English Language (according to the CED). 43 of those are from "Hamlet". In 1605, the total is 349: "King Lear and "Macbeth" account for 45 of that figure.
Shakespeare also contributed several (now) very well-known phrases which have become a part of the idiom. Have a look at the link below for some examples. Shakespeare's contribution - however you look at it - to English is huge.
Chaucer, many years before Shakespeare, was one of the first writers to exploit the variety and diversity of English. He exploits French, Latin, Greek, Arabic root words and concepts and brings them into English: he was also one of the first poets to write in colloquial everyday speech (read, for example "THe Reeve's Tale") and to bring "normal" speech into literature. Many, many examples of "first appearances" of foreign-borrowed words appear in Chaucer.