I've been reading up on loquats. They apparently are a hardy easy going tree that survives neglect.
Actually if the trees are about the same age and the fruit of both of them are good, I would be more concerned about the fruitful sparse tree.
Around here tropical fruit trees abound in many older neighborhoods. (Newer neighborhoods have HOA rules that limit fruit trees and the yards are a lot smaller). Most people complain that a once productive tree has stopped giving fruit. Here are the most frequent causes. Maybe some of these might be true in your situation.
1. The tree was giving a lot of fruit, but the owner never fed or watered the tree and the grass in the yard is old compacted, tired, sparse, weedy because they were not fertilizing or watering the grass either. Feed the tree and apply compost to help with the compacted soil, so tree roots have a place to go.
2. A good fruit producing tree was getting too big or hanging over the fence, so the owner severely pruned it (more than half of the tree being taken off at once, now the tree is making leaves but has not fruited for 2 years or giving only a couple of fruit if they are lucky. The severe pruning of the tree, shocked it, the tree will try to replace the canopy channeling its energy into growth and not fruit production until the canopy is restored which can take 2-5 years depending on how much canopy needs to restored and if the tree is being fed and watered. Some trees do not like to be pruned loquat and citrus only need minor pruning. No more than 1/3 of a tree or should be pruned off at one time. If more needs to be taken off, prune in stages and preserve fruiting wood.
3. The tree, usually a citrus, has given an abundance of fruit for years, more than "normal", but now the tree is sparse, produces a lot of fruit, but the quality is bad. This is usually a sign of a tree dying of a virus. Early in the infection the tree will try to survive by putting out more fruit than normal (trying to reproduce), as the infection progresses the tree tries to maintain production to survive but does not get enough food to produce quality fruit, eventually the tree will die of the disease.
Happy gardening in Hawaii. Gardens are where people grow.