I would not take these numbers as granted, since ancient sources almost invaribly exaggerated. It's not rare to read in ancient Chinese texts that armies could be up to one-million-man strong. However, I do agree that Han dynasty was able to field a considerable cavalry force to answer the threats from the nomadic Xiongnu empire.
If these large forces would be effective against Roman legions, is another matter. Countless times, Rome defeated enemies with much superior cavalry forces. In many battles of the Mithridatic wars, most notably Tigranocerta, relatively small Roman force soundly defeated their Armenian/ Pontic enemies with 50,000 of cavalry. It indicated that the bulk of these cavalrymen were light cavalry, useful for scouting and raiding but less so in battlefield. AFAIK, China didn't use armoured heavy cavalry in large number until the period of Three Kindoms.