Why recovery improves when calories are higher
Training volume, often measured as hard sets per muscle group per week, is a major driver of hypertrophy, but the volume a lifter can productively recover from is capped by recovery capacity. An energy surplus tends to raise that cap: more available glycogen, more raw substrate for tissue repair, and generally better sleep and mood all support faster recovery between sessions.
This is why many lifters find they can add sets and sessions during a bulk that would feel unsustainable while dieting. The body is not fighting an energy shortage, so the cost of each extra set is easier to absorb.
Volume limits still apply
A surplus widens the recoverable window but does not remove it. There is still a point where additional volume produces more fatigue than adaptation, and junk volume that is too light or too sloppy adds stress without much stimulus. Progressive, gradual increases monitored against performance and joint comfort work better than jumping volume sharply just because calories are higher.
The percentage figures often quoted for how much more volume is tolerable in a surplus are rough generalizations from training experience, not precise clinical values. This is general educational information, not medical or coaching advice for any specific person.