Why do my recorded vocals in GarageBand sound low on my system whereas they sound loud when heard via Focusrite through headphones or external monitors?

Good evening from India. My usage of GarageBand is very limited: I have music tracks for my songs composed by my music composer who gives me the song's track. I then import this track into GarageBand and record my vocals in my own home studio. What puzzles me is this: as I go ahead with finalising the mix, the sound levels are quite good when I hear them through headphones or external monitors, but, when I hear through the system's speakers, they become quite low. I am unable to increase the sound level further because the meters begin to turn red. I would like someone to guide me in this matter.

Regards

Dr. Somanna