true. but, we were coming from translations mainly urdu/english and elder is kabeer or akbar interchangeably; hence the jump. also if you check in lisan al-arab and taj al-arus you will notice that akbar is used as a substitute (in the meaning of) kabeer/elder. however, if we are talking of correct and exact plurals in arabic, you are right. -- taj al-arus: plural of akbar is akaabir or akbaruun. also see surah al-an'am, v.123. -- the plural of kabir as kubaraa is found in surah al-aHzab, v.67. and in taj al-arus, there are a few more: kibaar (commonly used in urdu as well), kubbaaruun, makbuura'a. in lisan al-arab: feminine of akbar is kubraa; plural of kubraa is kubar as in surah al-mudatthir, v.35. -- i am yet to find akabireen. Allah ta'ala knows best.