The best way to think about expansion of a company is to break it down into different factors: Consumers buy more clothes per visit, they buy clothes more often, they spend more per item of clothing, get new cosumers to purchase or buy Zara goods for a different occasion.
1 - More clothes per visit - Zara has a well know supply chain that can have new items in store almost every day, encouraging people to buy more goods. In addition they could run promotions to encourage more purchases such as buy 3 get 1 free.
2 - Buy clothes more often - By expanding online consumers don;t have to go to the high street so may purchase clothe more frequently. Furthermore the new styles coming in encourages people to visit the store frequently so encouraging this type of growth.
3 - Spend more per item - this could involve making the brand appear more premium through advertising, or by having a more premium version (they do this by having other stores such as Massimo Dutti).
4 - New consumers - Zara attempts to do this by covering a range of age groups through Brands like Massimo Dutti and Pull & Bear.
5 - New occasions - Zara already has a home store so people can purchase furniture, fragrances etc. They also try to cover most types of clothes from sport to formal, but could theoretically do this more - ski gear, work wear etc.