Hello Valentin,
For an unknown reason (at least unknown to me), LilyPond's default
beam-thickness is 0.48 staff spaces.
The common standard, however, seems to be 0.5 staff spaces (Finale, Sibelius,
Dorico, ...) and, most importantly, Elaine Gould ("Behind Bars") suggests 0.5
staff spaces.
Classic engravings tend to use even much thicker beams.
I don't know whether LilyPond's standard should be changed, and everyone is
free to change LilyPond's beam thickness by using
\override Beam.beam-thickness = #...
Cheerio,
Torsten
