"Felony" is a legal term -- and the "seriousness" is related to the laws passed and the culture around them. Heck, it might not even reflect society's opinion of the offense.

Back in the day stealing a car for a ride (equally a "felony" as stealing it permanently) was something "delinquent youth" did routinely, without much harm to anyone -- the practice, common in many countries, is called a "joyride" (nowadays electronic locks make it more difficult -- it was much more prevalent back in the day).