Compared to an FLT-100, both the Oscar-B and ROUND-8 contain far too much fuel for their size.
The volume of an Oscar-B for example is roughly about 1/6 that of the FLT-100, yet it contains 2/5 of the fuel.
The fuel and corresponding weight of both tanks should probably be adjusted to roughly half their current values, something like 1/5 - 1/6 of the FLT100 for the Oscar-B and 1/3 - 3/10 of an FLT100 for the ROUND-8.