I think its most likely that range was the issue here. As prievously said you can see the landscape of Katarr quite clearly, suggesting he was very close. The size of the planet is irrelevant Legend, even if we pretended your argument wasn't pure conjecture basing itself what is ultimately an artists interpretation and far from accurate evidence, you've still have to be extremely close to make out mountains and structures, even if the planet were moon sized.
However more importantly, the Ravager was in orbit above Telos for a pretty long time, likely hours, enough for the Exile to battle her way through the station, board a transport, battle her way through the Ravager and reach the bridge.
By Legends argument, it therefore takes hours for Nihilus to gather the strength to absorb a planet, which suggests he was in orbit over Katarr for that amount of time. Why is this problematic? Because nobody reacted, picture the scene, the Jedi Order is a crisis, they have been forced into hiding by the Sith, in a desperate act they arrange a secret conclave in attempt to salvage the Jedi Order. A massive Sith warship appears in orbit and proceeds to sit there.
What do the Jedi do in this situation? Sit and wait for the fireworks? No, they get the hell out of there. Simply put Nihilus' attack would have to have come hard and fast otherwise the Jedi would have fled. And yet they didn't and all source point to him striking without warning, and insta-drained the planet.
Given that, the only thing that could have stopped Nihilus consuming Telos, was range, he had to dispense with the Republic fleet before gobbling the planet.