A head to head comparison of the Illinois 2016 Primary gives us an idea of the break down of the voters.
Hillary Clinton received 1,o17,006 (30%) votes beating Donald Trump, who received 556,916 votes (16%), by 460,090 votes.
In the primary, Trump won 61 counties in the 4-way match up. Clinton trailed behind Sanders with 6 counties. Cruz and Sanders won 19 and 16 counties respectively in Illinois Primary.
Bernie Sanders vs Clinton
Hillary Clinton vs Donald Trump
Donald Trump vs Bernie Sanders
In a head to head match up on just the total number of votes, Hillary Clinton (51%) beats Bernie Sanders (49%). Sanders (64%) beats Trump (36%). Also, Clinton beats Trump with a 65% over his 35%.
These 10 counties play an important role in these primary elections. If a candidate, can get a sizable majority in the three larger counties (Cook, DuPage and Lake), it becomes very difficult to win majorities in the smaller counties with out voter out reach and increased voter participation in them.
Hillary Clinton's top 10 counties of votes came from the top 10 vote size counties. She ran the board on Donald Trump in every one of them which could have contributed to such a lopsided result.
Cook, which is the biggest county voted very favorably for Clinton, giving her over 400k vote advantage out of that county. Winning many other important counties, including DuPage, Lake and Will made Clinton gain an upper hand on Illinois.
Related References: 2016 Illinois General Election Story, 2020 Illinois Primary Election Story