減量 refers to tangible things you can weigh or count - objects in the real world which have weight.
減少 is a broader term which includes physical objects but also abstract concepts.
The key to understanding the subtle difference lies in the kanji. Of course the kanji
減 remains constant, so the difference is between
少 (few, little) and
量 (amount, quantity).
When we check definitions of these words, we can see that
減量 (definition) tends to refer to actual physical things, like reducing a quantity of objects, reducing body weight, reducing the weight of something in a recipe. On the other hand,
減少 (definition) is a more general word which is a reduction in anything. This might include actual physical objects as above, but it also includes abstract reductions as well, such as a reduction in working hours or a decrease in population.
The same principle applies to
増量 (an increase in quantity or weight) and
増加 (a general increase including abstract concepts). There are many words in Japanese which have similar meanings, but often the kanji can give us subtle clues as to what the deeper distinctions between them are.
So always check the kanji :)