Largely, greatly, mostly, chiefly, mainly, principally, generally are often interchanged, but they are capable of being used with explicitness even though they basically agree in meaning.
Largely stresses quantity or extent; it usually connotes copiousness or abundance and often suggests an amount exceeding that of other ingredients, components, or constituents.
Greatly carries a heightened suggestion of greatness in degree that differentiates it from largely.
Mostly usually stresses numbers.
Chiefly emphasizes the importance of one thing among other things; it may connote an outstanding or preeminent position or it may connote merely relative importance <the basket contained many fruits, but chiefly apples> <the battle was won chiefly by the aid of the air force>
Mainly is often used interchangeably with chiefly, but not where preeminence is implied; rather it connotes greatest importance among a number of things, but not exclusive value <be sure to take along with you all that you will need, but mainly informal clothes> <the cause depends mainly on the validity of this act —John Marshall > <the Pickwickian Christmas did very little to stimulate consumption; it was mainly a gratuitous festivity —Huxley >
Principally carries an idea of primary importance rather than outstanding or relative importance; the difference is not great between it and chiefly or mainly except when the idea of being first or primary is emphasized <they wholly mistake the nature of criticism, who think its business is principally to find fault —Dryden > <his support comes principally from the income of invested money> <the cash crops are principally wheat and rye>
Generally stresses reference to the majority of persons, instances, or cases involved <the people, not universally, but generally, were animated by a true spirit of sacrifice —Froude > <the news was generally received with joy> <the land breezes here are generally hot and dry>