Do we have any idea what the redshirt contingent was of a TOS-era Constitution Class starship? If redshirts accounted for more of the crew than any other group, it shouldn't be so alarming that the number of overall losses reflect that ratio.
When we talk about "redshirts," we really mean security guards. Red was the color for engineering and operations as well as security, but the vast majority of the "redshirt" fatalities were security personnel. It stands to reason that security guards would be the first in harm's way and thus have the highest casualty rates. Their job is to risk -- and, if necessary, sacrifice -- their lives to protect the rest of the crew.