I designed these as an exercise to see how small I could make a bracket while still being strong enough for a large shelf.
While I don't know the actual breaking point of these (the chipboard test I did broke), I put 15KG's on a 122cm x 20cm pine shelf with 2 brackets for a few days and can see no signs of stress.
Basically, for the size, these are extremely strong, just maybe don't put 30 rolls of filament on it.
The main single wall hole is 4mm across. The 3 holes into the shelf are 3mm across.