When the safety briefing on an airline flight tells you to "fasten your seatbelt low and tight around your waist," don't bother with the "low and tight" part. Make it as loose as you can, and you'll be more comfortable. Flight attendants don't check the tightness, only that it's buckled.
Compared to a car, the safety value of an airplane seatbelt is negligible. The likelihood of you needing it is extremely thin. Air travel is all about safety overkill.