In untrained individuals, pre-existing lean body mass has minimal predictive value for the hypertrophic response to resistance training, as indicated by correlation coefficients ranging from 0.1 to 0.2.

Study: Muscle Transcriptional Networks Linked to Resistance Exercise Training Hypertrophic Response Heterogeneity.

The study looked at how people's muscles respond to weight training and found that what their bodies are like before starting doesn't strongly predict how much they'll gain, which supports the idea that starting muscle size isn't a great predictor of growth.

Study: Resistance training-induced appendicular lean tissue mass changes are largely unrelated to pre-training bone characteristics in a larger cohort of untrained adults

The study looked at how much muscle people gained from weight training and whether starting with more muscle helped predict gains. It found that how much muscle you start with doesn’t really predict how much you’ll gain.