Is Protein-to-DNA ratio a good estimate of cell size?
So after discussing my concerns with my professor, I have found out that I overlooked the simple fact that the DNA concentration is not measured based on a single cell of tissue type. 

Protein-to-DNA ratio is directly proportional to cell size and the reason why liver cells has the highest DNA concentration at 57.6 ug/ml is because they have a smaller cellular size compared to the other two. So a single ml of liver homogenate is going to present with a significantly higher cell count and thus, a higher DNA concentration. This is also why a single ml of heart muscle DNA concentration is only 8.6 ug/ml due to there much larger cellular size.

This finding can be confirmed from calculating the Protein:DNA ratio as the liver homogenate with its smaller cellular size is only 0.184 and the heart homogenate with its larger cellular size is the highest at 0.548.

Thank you everyone for trying to help me with this problem!