This is heavy. There are lots of people here.

When most people talk about things as being heavy or a large number of objects they usually use 「重い」 (heavy) or 「いっぱい」 (many, lots). These terms are fine, but not always the most accurate. In some cases, I prefer to be able to describe things in terms of mass or density. The terms 「質量」 and 「密度」 are easy enough to look up in the dictionary, but maybe a little tricky to learn how to use because you don't hear them used often in normal conversation. So hear are a few examples.

• 英文：massive part
• 和文：質量の多い部品
• 英文：dense part
• 和文：密度の高い部品
• 英文：That front end has more mass than the back
• 和文：前の部分が後ろより質量が多い
• 英文：His hair density is low but he's not bald.
• 和文： 彼の髪の毛の密度が少ない、でも彼ははげてない。
• 英文：These two parts are the same size but this one has denser mass than the other.
• 和文： この２つの部品の大きさは同じですがこの部品の質量密度のほうが高い

The things to remember are that 「質量」 is usually quantified by 「多い」 and 「少ない」 and 「密度」 is quantified by 「高い」 and 「低い」. 「密度」 can also be quantified by prepending 「高・低」 resulting in 「高密度（こうみつど）」 or 「低密度（ていみつど）」.

On a practical note, I don't need to use 「質量」 all that often, but I use 「密度」 just about every day.