The principles are the same, though. The developer phase is quite critical. Temperature fluctuations of even 1 degree F. can very strongly impact the overall density and contrast range of the final results. Depleted, exhausted developer. Inadequate agitation during the developer phase of processing.
Exposures have to be increased for bigger, denser feet.
My mule's feet turned out to be weaker exposures than my horse's for this reason. Outdated x-ray film can have an overall dichroic fog, lowering overall contrast. Film that is not outdated can still age prematurely if it not kept cool. Factors that result in dense, overly contrasty negatives are the opposite of the above. In addition, old developer that has been over-replenished with developer replenisher results in too contrasty a result.