A head to head comparison of the 2012 Georgia General Election gives us an idea of the break down of the voters.    

Mitt Romney received 2,070,220 (54%) votes beating Barack Obama who gained 1,761,760 (46%) by 308,460 votes in Georgia.

Mitt Romney won 125 counties whereas Barack Obama won 34 counties in the total of the 159 counties in the State of Georgia.

Presidential General Elections

During the 2012 General elections, over 122M total number of votes were cast. Barack Obama got 62,614,654 (62.6 Million) over Mitt Romney's 59,141,654 (59.1 Million). He won the popular vote by 3,472,979 (3.4 Million). Even though he lost to Mitt Romney in the State of Georgia, his 1,761,760 gained votes represents 2.7% of his popular vote victory. The remaining 49 states contributed to the remaining 97.3% popular votes victory.

Top 15 Counties in Primary Elections vs General Elections

All Candidates (General Elections)

These 15 counties play an important role in these Primary elections. If a candidate, can get a sizable majority in the largest counties (Fulton, Cobb, Dekalb, Gwinnett), it becomes very difficult to win majorities in the smaller counties without voter outreach and increased voter participation in them. However, steady win margins in less popular counties can amount the a marginal win the an entire State. (As in the case of Mitt Romney in the State of Georgia)

Top 15 Counties Comparison in Primary Elections vs General Elections

General Elections
Barack Obama

General Elections
Mitt Romney

Fulton and Dekalb, two of the larger counties voted in favor of Barack Obama, giving him a 288K vote advantage. On the other hand, Cobb and Gwinnett, other larger counties, voted in favor of Mitt Romney giving him a 66K voter advantage. Referencing the above table, a summation and comparison of the top 15 counties for both candidates indicates a 158K vote advantage for Barack Obama. However, steady marginal wins for Mitt Romney in the less popular counties gave me an upper hand in the State of Georgia.

Related References: 2016 Georgia Primary Election Story, 2016 Georgia General Election Story,  2020 Georgia Primary Election Story