The size of the balloon is increasing.
Many students are familiar with the word increase, but fail to use it correctly. Try to spot the error in the following sentence:
Elderly people are increasing in Japan.
Increasing means becoming larger, so if people are increasing, perhaps each person is getting larger. Maybe they are getting fat. This use of increase is not only unclear; it paints a very strange picture.
You should say,
The percentage of elderly people is increasing in Japan.
The percentage of elderly people is getting larger, or there are more elderly people compared to young people.
In fact, many native English speakers do make grammatical mistakes. Some may even misuse "increase." However, native speakers already have a trusted language ability. Their natural rhythm and confidence with language typically cause the listener to overlook small grammatical errors.
On the other hand, non-native speakers who use slightly unnatural intonation, pronunciation or word choice will have to work harder to gain the listener's trust. One way of gaining that trust is to use correct grammar.
Of course, speaking tests also score grammar. If you are planning on taking any English test, particularly a speaking test, then be careful to use increase correctly.
So just remember: numbers and amounts increase. General nouns like prices, consumption, tax rates and the like can also increase.
If people are increasing, however, we should be worried, or at least consult a doctor.
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paints a very strange picture [idiom] - it means to give the listener an clear image to think about, in this case it's very strange.