If you mean большой-большою, it is not unlike "I'm" and "I am". A phonetical contraction. The difference is that the full form is rare. Mostly it is used in poetry. Occasionaly in speech, just by taste. Mostly consciously, not many people can use it spontaneously. Usually in adjectives. In 19 century it was more common. People may use it - for stylization (as it is found in poetry it now sounds like a 'poetic' form) - just because they read classical literature and poetry and see it often. - in "ею" to distinguish it from dative "ей".

There is always only one option for each word (mama -> mamoy). It is just that the ending will be different within the same declension if in the nominative the endings are different. So you just gotta be sure in what declension you are.