Even without his ability on, Malachi could see the amount of red gas rushing into David's body and knew what they would be doing in his system.
For some reason, David chose to absorb the monster dog's crystal heart last. Perhaps his intention was to save the best for last.
Malachi expected and knew that by the time David was done absorbing this monster's crystal heart, his strength would reach a different level.
Perhaps David might even be stronger than him, but Malachi did not know for sure.
If it was in terms of ability, then David would win with no competition due to the damage his ability could cause.
What Malachi was unsure of was if David would be stronger than him physically.
If one were to put aside the monster dog heart that David was currently absorbing and compare the overall numbers of crystal hearts they had absorbed, Malachi could not see himself losing to David's physique by a big margin.