I see this statement was made 10 months ago. By now, you have probably realized that the hover words should not be thought of as suggestions, but merely clues as to what words may possibly be used. Many of the words shown don't work in the context of the given sentence, but work wonderfully in other contexts.
When I search the phrase "cada vez mayor" in SpanishDict, I found a list of translations around a theme of increasing. I'm thinking this might just be one of those cases where a phrase has an understood meaning outside of the literal word-by-word translation. English has phrases like this as well. We, as native English speakers, just don't notice them because we're used to them. I started tutoring English to a non-native speaker a while back and it has opened my eyes to this quite a bit.
Here's the link in case it helps anyone else. It helped me get a general sense of what this string of words can do in Spanish at least.
Context will tell you the meaning. Since there is no context, both can be correct. They are vaguely connected in meaning if you think about it in that sense, though. Think about "older brother", for example. Tiene tres hermanos mayores (she has three older brothers). They're bigger than her (in age). They're older brothers. They're bigger brothers (in age). The population is getting bigger (in numbers). The population is getting older (bigger in age).
The English translation implies that the population is getting fatter. If you want to indicate the number of people is increasing, which I think the Spanish sentence indicates, we'd say "The population is increasing" or "The population is growing." I wrote the latter and was marked wrong. Duo needs to correct this.