For such a thin skeletoned man, he has a large cranium, and presumably a sizable brain. It’s too bad he did not use it for good, and instead turned his intellect to the “task or propaganda”, or deceiving his own countrymen.
I think his slight brain simply made his head look bigger. Same with dolphins having greater brain to body ratio than humans but the dolphins don’t have arms or legs. His rise can be attributed entirely to his ambition and ruthlessness.