The size of the average warehouse completed this year was 188,000 square feet, more than double the size in 2001. They are also 21 percent higher.

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Those boxes piling up on your doorstep over the holidays don’t ship from Santa’s workshop. As Americans spend more money shopping online, real-estate developers are sinking record amounts of money into new warehouse space, building bigger, taller structures to meet the needs of e-commerce — and the robots that help it along.

Builders spent $2.7 billion on U.S. warehouse construction in October, the most since the census started keeping track in 1993.

The size of the average warehouse completed this year was 188,000 square feet, according to a report published last week by CBRE Group, more than double the size in 2001.

Developers are also raising their roofs, with ceiling heights up 21 percent over that period.

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Warehouses are getting bigger for the same reason retailers and logistics firms are building more of them.

“It’s the notion of the endless aisle,” said Joe Dunlap, a managing director at CBRE. A retailer that stocks 30,000 items in its stores might offer 10 times as many items for sale online. More stock requires larger footprints. Higher ceilings accommodate mezzanine levels, letting operators cram more shelves into a building.

Today’s industrial buildings also require thicker concrete floors to support heavy machinery used to automate the warehouse.

Warehouses are likely to keep getting bigger, and more expensive.

“Long term, there has to be a landing point,” Dunlap said, “probably driven by finding the right collaboration” between robots and people.