Massive, massy, bulky, monumental, substantial are comparable when they mean impressively large or heavy.
Massive distinctively stresses solidity and strength of construction and may imply an imposing appearance.
Massy, chiefly a literary word, carries a stronger implication of ponderousness than massive, but an equal implication of solidity and strength.
Bulky stresses size rather than weight, and the excessive amount of space occupied rather than solidity or strength of construction.
Monumental implies greatness of size, but it distinctively suggests an imposing massiveness.
Substantial stresses solidity and strength of construction but it carries a weak implication of size or outwardly imposing appearance and a strong implication of established quality, worth, and stability.