Fleet carriers are big and they cause balance issues in Elite Dangerous. Not unfixable ones, mind you; just ones that have to be adjusted over time. For example, one of the existing problems in place was that overlapping hotspots for resources were overwhelmingly powerful, so that was patched to no longer be the case and ensure that single hotspots are useful, while overlapping ones are nice but not as transformative. The flip side? Tritium became much harder to get.
This meant that getting around the galaxy was more tedious than the designers wanted, so another change is incoming to double the effectiveness of tritium. As a result, fleet carriers will now consume half as much fuel, making them more useful even as the quantities mined remain on the lower side. Player opinions on the change are mixed at this point, with some happy at the increased utility and others miffed at the lowered importance of deep-space mining, but it does provide some insight into how these systems are meant to work regardless.