重い and 軽い: weight, burden, and severity
See how the physical opposites 重い and 軽い extend to responsibility, symptoms, exercise, and attitude. The surrounding noun shows whether the sentence concerns weight, burden, intensity, or severity.
Heavy or light in physical weight
重い means that an object has substantial weight and may be difficult to lift or carry. この箱は一人で運ぶには重いです。 directly evaluates the box's physical weight.
軽い describes low weight and ease of handling. このかばんは軽いので旅行に便利です。 is about a lightweight bag. On the same scale, この箱は重いが、あの箱は軽いです。 forms a straightforward contrast.
The burden of responsibility or an activity
重い責任 and 重い決断 describe a responsibility or decision with great burden and importance. 重い責任を任され、彼は表情を引き締めました。 involves no measurable physical weight.
軽い運動 and 軽い食事 describe activities or meals that place a relatively small burden on the body. In 毎朝、軽い運動をしてから仕事を始めます。, 軽い primarily means low in intensity, not necessarily short in duration.
Serious or mild conditions
病気が重い and 重い症状 can describe a serious condition. By contrast, 症状が軽い means that symptoms are mild. 医者は症状が軽いので入院は必要ないと言いました。 makes the scale of severity clear through the result.
軽い気持ち can also suggest a casual attitude or insufficient seriousness, so 軽い is not automatically praise. Use the noun and the full sentence to determine whether 重い and 軽い refer to physical weight, seriousness, burden, or intensity.
Quick check: In 医者は症状が軽いので入院は必要ないと言いました, does 軽い refer to low body weight?
Show the answer and why
No. It says that the symptoms are mild rather than serious. The adjective evaluates the severity of a condition, not the physical weight of a person.