Straightforward question. The Pacific Ocean is
太平洋【たい・へい・よう】 and the Atlantic Ocean is
大西洋【たい・せい・よう】. What was the reasoning in using a different character for the
たい in each? And why
太 for Pacific and
大 for Atlantic?
As the characters clearly suggest, 大西洋 simply refers to the big Western ocean. Not much confusion there.
太平洋, however, is an adaptation of the English "Pacific" Ocean. 太平 is a word in its own right that means roughly this: "peaceful" or "tranquil," or "pacific," if you will. According to the page linked at the bottom it had previously been known as 大東海.
It seems like 太 may be a simplified form of 泰, although apparently this isn't listed in all character dictionaries (note that here a person claims to have found it). The word 泰平 exists and is the same word as 太平, and based purely on shape it makes sense that one would be a 略字. So if this theory is to be believed then the 太 in 太平洋 does not mean thick/fat, but rather peace as in 泰平, because 泰 simplifies to 太, and is not to be confused with Thailand.
Lengthy discussion about it can be found here! http://homepage2.nifty.com/osiete/s539.htm