difference in volume between aux out/main volume

my mackie mixer has main outs and aux outs and when i send something through the aux outs, the volume is very low compared to the main out level...how come?

when i record on my sampler that is connected to the aux outs and i normalize the signal after recording, there is a terrible amount of "hiss", compared to when i hear something out of the main outs, there is no "hiss" at all.

how come there is such a volume difference between the main outs and aux out?

First, what model mackie mixer is it that you're working with? Second, usually "aux outs" are either sent to monitor mixes in some cases or outboard efx and whatnot. And there's usually some type of adjustment control on the board pertaining to this.

So the volume in this case is irrelevant to being equal to the main outs because of other intended purposes.

If the AuxSend and the AuxMaster are all the way up and still not as loud as the Master it could be that the Aux work in postFader mode. If you can set it to preFader, do so.
Or (and this is a more recommend solution) you readjust the complete signal path.

Use a reference tone from the source and adjust the gain on the mixer channel with the PFL turned on till it hit the 0dB LED,
turn off the PFL, set the channel fader to 0dB and then set the master fader so the LED hit the 0dB.
Then turn up the volume on your speakers till you think its loud enough.

As long as you don't turn down the channelfader you should have a good signal on the Master AND the Aux (with AuxSend AND AuxMaster turned up)

peace