22x58 DC fuse holder getting HOT

Hi all, I'm having a bit of an issue with my setup and I'm not quite sure whether it's normal.

I started with a 3kVA/3kWp system and have been slowly upgrading (now 5kVA/5.4kWp). When I fitted the new inverter over December I noticed that some of the wires near the PV combiner were getting uncomfortably hot, and the bigger inverter was just making things much, much worse.

Most of the heat was coming from the 22x58 fuse between the (Victron) MPPT/Charge Controller and the batteries, but almost everything in the combiner box was heating up badly. What I've done so far is:

1. replace the 22x58 fuse holder & fuse (Onesto -> Inge)
2. rewire everything after the 10x38 PV string fuses. 16mm2 (was originally 6mm2) cable for the combined PV (carries < 40A at max load), and 35mm2 (was originally 2x6mm2) cable between the MPPT and battery (+ve goes through the 22x58 fuse that heats up - current is around 70A at max)
3. Tighten/retighten all the connections (bare wire, no lugs)
4. Measure voltage drop over the various components

From my measurements (see 4 above) it looks like the MPPT-Battery fuse is dissipating around 6W, which from looking around seems to be about normal (e.g. Victron 125A/58V mega fuse lists voltage drop of 90mV, which at 70A would also give roughly 6W dissipation). I guess because of poor ventilation in the combiner box heat buildup is to be expected, but it "feels" like something isn't quite right with how hot the cables and fuse holder get. I just don't really have any practical experience with DC stuff like this to know whether it's to be expected or if I should keep looking for another possible cause/problem with the setup.

Can anyone maybe offer some guidance? Should the fuse holder really be getting that hot, or does it sound like there's still a problem somewhere and if so, what should I be looking for?

Thanks for any input