Pictures all over the place confirm that arapaima gigas grow much larger than the claimed 2.5M in this thread. Several in my albums are 3 M plus.
Why do I think they grow as large as I claim? Because I've seen them.
I have also seen many self proclaimed "keepers" housing them in terrible conditions more than I have seen them housed correctly. These sub-standard aquariums/pools lead to unhealthy stunted animals. When 80% are kept wrong, suddenly 80% are showing sized that are un-natural. Suddenly, the 'normal' sized fish (3M or more) are becoming the 'freaks' only because keepers are not allowing them to grow correctly.