A while ago there was a patch for Net-SNMP 5.5 which introduced a new option realStorageUnits for the configuration file.
From the Redhat Bugreport #748410:
To address this issue [negative hrStorageSite values], this update adds a new option to the
/etc/snmp/snmpd.conf configuration file, realStorageUnits. By changing
the value of this option to 0, users can now enable recalculating all
values in hrStorageTable to ensure that the multiplication of
hrStorageSize and hrStorageAllocationUnits always produces an accurate
device size.
(text in [brackets] is mine)
So adding the configuration directive realStorageUnits 0 to your snmpd.conf might be solving your problem.
However, the values will not be correct up to the very last megabyte; ymmv.
I can’t tell if this patch was included in your binary distribution of Net-SNMP, but it would be great if you could report the results and what binary you are using. Also, I didn’t test it for the lack of adequate hardware right now.