Something is 【heavy】 which is denser and more compact in substance or larger in size or amount than the average of its kind or class and so weighs more in proportion.

In extended use what is  【heavy】 weighs down the senses or the spirits or is of such nature that the mind or the body finds difficult to bear or endure.

Often, also,  【heavy】 is applied to the heart, the mind, or the body to imply a being weighed down (as with grief, worry, weariness, or overwork).

At other times the term merely implies a lack of some quality (as lightness, vivacity, or grace) which enlivens and stimulates.

Something is  【weighty】 which is actually and not merely relatively 【heavy】 . In extended use what is  【weighty】 is highly important or momentous or produces a powerful effect or exerts an impressive influence.

Something is  【ponderous】 which is exceedingly 【heavy】 because of its size or its massiveness and cannot move or be moved quickly. In extended use what is  【ponderous】 is unduly intricate, involved, complicated, or labored.

Something is  【cumbrous】  or  【cumbersome】 which is so 【heavy】 and so bulky that it is difficult to deal with (as in moving or carrying). In extended use both words are applicable to what is both 【ponderous】 and unwieldy.

Something is  【hefty】 which one estimates as 【heavy】 or 【weighty】 (as by holding in one′s hands or by measuring with one′s eyes). In extended use the word may imply a generous amount or portion.