As robots pack more and more items into Amazon's fulfillment centers, reaching high up on shelves to store and retrieve toiletries, electronics and household goods, the company is reconsidering how it accounts for space.

On last week's earnings call with analysts, Chief Financial Officer Brian Olsavsky said that Amazon could start using cubic feet, instead of square footage, to measure its warehouse footprint. That would change the measurement from one of sheer size to one of volume, and give a clearer picture of how Amazon is becoming more efficient as its inventory grows.

"We are debating whether the dynamics of the warehouse are changing, so that square footage may not be the main indicator — it might be cubic feet," Olsavsky said. He said that if the company makes a change it will provide more detail to investors, but he didn't say when a decision will come.

As of now, Amazon expects its warehouse footage space to grow 15 percent in 2018, slower than the 30 percent growth rate over the prior two years. At the end of 2017, Amazon occupied almost 200 million square feet of space in its warehouses and data centers.

The company is moving in the direction of vertical expansion, filling up multi-story warehouses, which is particularly important as it builds in more densely populated urban areas. So its actual use of a warehouse is no longer shown by just length times width but increasingly includes height.

"Amazon is getting more and more efficient at using the warehouse space in a three-dimensional way," said Suvrat Dhanorkar, a supply chain management professor at Penn State University.