Why a small surplus beats a large one
A lean bulk works on a simple premise: muscle protein synthesis has a ceiling, so calories above what is needed to support growth tend to be stored as fat rather than tissue. A modest surplus of roughly 200 to 300 kcal per day supplies the building materials and recovery energy without overshooting that ceiling by much.
Targeting around 0.25 to 0.5 percent of bodyweight gained per week is a common guideline for trained lifters. For an 80 kg person that is roughly 200 to 400 g per week. Faster gain is possible but tends to carry a higher fat fraction, which then has to be dieted off later.
The trade-off between speed and fat gain
Genuine novices and people returning after a long layoff can often grow muscle faster and partition more of a surplus toward lean tissue, so a slightly larger surplus may be reasonable for them. For intermediate and advanced lifters the rate of muscle accrual is slower, so a tighter surplus keeps the fat-to-muscle ratio of the gain more favorable.
Individual response varies with genetics, sleep, training quality, and protein intake, so the numbers here are starting points rather than fixed prescriptions. Tracking weekly average bodyweight and adjusting calories up or down based on the actual trend is more reliable than assuming any single figure applies. This is general educational information, not medical or dietary advice; consult a qualified professional for individualized guidance.