1956 -- Eisenhower -- 5,008,132 --- out of 5,828,434
1960 -- Nixon -- 4,975,938 --- out of 5,743,634
1964 -- Goldwater -- 2,267,079 --- out of 5,514,527
1968 -- Nixon --- 1,679,443 --- out of 4,473,251 (Reagan received 1,696,632) (15 states)
1972 -- Nixon --- 5,378,704 --- out of 5,828,483 (18 states)
1976 -- Ford --- 5,529,899 -- Reagan --- 4,760,222 (First time primaries or caucuses held in every state)
1980 -- Reagan --- 7,709,793 --- out of 12,850,432 -- (60.0%)
1984 -- Reagan --- 6,484,987 --- 98.78%
1988 -- Bush --- 8,253,512 --- out of 12,851,740 -- (64.2%)
1992 -- Bush --- 9,199,463 --- out of 12,596,601 -- (73.0%)
1996 -- Dole --- 9,024,742 --- out of 15,313,343 -- (58.9%)
2000 -- Bush --- 12,034,676 --- out of 19,391,600 -- (62.1%)
2004 -- Bush --- 7,853,863 --- 98.1%
2008 -- McCain --- 9,840,746 --- out of 20,828,435 -- (47.3%)
2012 -- Romney --- 9,947,433 --- out of 18,908,313 -- (52.6%)
2016 -- Trump --- 10,717,357 --- out of 25,731,093 -- (41.7%)

Population in 1956 was nearly 168.9 million
Population in 1976 was nearly 218.0 million
Population in 2016 is nearly 322.7 million

Even using population to attempt a correlation would be wrong by virtue of not everyone even participating in the primaries and not everyone is a Republican. Even the ratio of Republicans changes through time.

What is evident is the weak support Trump has within the Republican Party compared to past elections.