Hi Vincenzo
Attached (or here) is a simplified version of the full script which reproduces the problem. You can test it on this file on CERNBox (user/e/egramsta/forROOT/user.egramsta.48785775._000523.output_ntup.root).
When I run this script in ROOT 6.36.04 the size/ev is 261.37 bytes (total size 10 MB) while with the ROOT built from master on Nov. 20th 2025 (6.39.01) it is 331.96 bytes/ev (13 MB).
LargeSizeDemoScript.py (1.5 KB)
best,
Eirik