How does doxy work?

*Species: feline
* Age: 1+ years
* Sex/Neuter status: altered
* Breed: DSH
* Body weight: 14+ lbs
* History: shelter animals
* Clinical signs: URI- sneezing, nasal/eye discharge
* Duration: 1-2 days
* Your general location:
* Links to any test results, X-rays, vet reports etc. that you have: n/a

I am a vet tech in a shelter. Can someone please explain how doxy works in treating cat URIs? I have a doctor who does doxy 10mg/kg with compounded liquid (50mg/ml), but if a cat is over 10# then they say dont give more than one ml. I feel like we are chronically under dosing our larger cats (14+lbs) and its taking longer for them to get better... Im not the only tech that notices this.

I dont want to say that I dont trust this doctor, but their choices do confuse me sometimes and their explanations arent always... great.

Thanks vets!