Sentences are largely set by the guidelines published by the United States Sentencing Commission and are based on a standard table[1], which assigns "points" based on the severity of the crime and level of involvement [0]. While it's plausible his sentence was harsh for "political reasons", it's more likely that's just the way the points added up based on the facts of the conviction.

[0] https://www.ussc.gov/guidelines

[1] https://www.ussc.gov/guidelines/2016-guidelines-manual/2016-...