I can't record my voice using GarageBand, but can still record instruments
I'm assuming the Instruments are already-loaded samples, you're just lining up along a track, or as a midi track to fire single note samples.  

If so, that requires very little additional storage - the samples are already there, it just needs to remember what order they go in. 

An actual audio recording, on the other hand, is going to eat about 5MB data per minute, mono, or 10MB/m stereo.