At the risk of stating the obvious, it's easier to play a loud mandolin quietly than to get a quiet mandolin to play loudly.

Most instruments have a limit. To try and put it simply, the sound which comes out of them all comprises an initial "thud" as the pick strikes the strings together with a "ring" as the strings sound out. At some point, the thud will overpower the ring and this occurs at a lower level on cheaper/poorer instruments which choke up if you try to play them too hard.

This may be an oversimplification as it also depends on string gauge, set-up and pick weight. Like Jon, I also use 1 - 1.4mm rounded picks (Wegen). I find that I need to go lighter and pointier on mandola and octave mandolin.