"My oranges are big."
It's a plural form in the English sentence, oranges instead of orange.
The Indonesian sentence needs to reflect that plurality, without being ambiguous.
"Jeruk saya besar" can be interpreted as singular.
"Jeruk-jeruk saya besar" can only be interpreted as plural.
If there is more context indicating the plurality, then you can leave out the reduplication while it's still obvious that you're talking about the plural form.
Something like this :
"Saya punya banyak jeruk, jeruk saya besar."
"I have many oranges, my oranges are big."
"Saya punya banyak jeruk, semua jeruk saya besar."
"I have many oranges, all my oranges are big."
You can reduplicate here as well, but it's not needed, because there is context indicating the plural form.
However, in the example sentence from the lesson, the only context is the English sentence.
So you need to reduplicate to be absolutely sure that the Indonesian sentence can only be interpreted as plural.
Without the reduplication, it can be interpreted as singular.