I think the medium is a great size for heavy items. I have packed it up with 5 lights of different sizes and speed rings, cables, everything. It weighs a lot and I’m glad I didn't get the large for lights… but I will probably be getting the large for modifiers and bulky lighter weight items.
Over all good bag, lots of dividers for customizing the layout is really nice. I wish it was just a couple inches deeper this way you could fit a 600 pro with a battery or an AC adaptor vertically. That would be very useful.
I will see how this bag holds up over years of abuse and load ins/outs.