Media playback is unsupported on your device Media caption Yang Hua is one of millions of couriers moving internet shopping around China

It doesn't take long to understand why Yang Hua is so thin.

Mr Yang is a delivery man, working long shifts for one of China's busiest courier companies.

Dwarfed by a huge canvas sack filled with envelopes and boxes, he moves as quickly as he possibly can from one floor to another in the office buildings where he works.

Mr Yang delivers packages to the office buildings located on a single block in downtown Beijing. It doesn't sound like much, but it's a job that, quite literally, keeps him on the run.

Image caption Yang Hua, a courier for ZTO, cannot afford to be late with his deliveries because he would be fined

If a customer calls to complain about a late delivery, Mr Yang's company fines him 100 yuan ($16; £10), almost all of his daily wages.

The fear of losing money on the job means he rarely slows down, rising at 06:00 to collect packages from a central warehouse and working until 19:00 in the evening.

"I can't even stop for a meal," he explains, without slowing his pace. "If you want to talk to me, we have to talk on the go."

Booming e-commerce

Yang is a front-line worker servicing China's booming courier industry - a sector that has emerged to service China's obsession with online shopping.

Chinese courier services will deliver an estimated 12 billion parcels this year, according to the government's postal bureau, putting China on course to surpass the US as the world's biggest delivery sector.

Image caption The growing popularity of online shopping in China means the warehouses of courier firms are always full

The vast majority of courier packages contain items ordered online. China has few brand-name bricks-and-mortar stores, giving millions of online retailers the opportunity to flog products at highly competitive prices.

The sheer size of China's online retail sector is staggering. Last month, on 11 November, the country marked Singles' Day - a relatively new holiday that was invented a few years ago to give unmarried people an excuse to spoil themselves a little.

On that day this year, Chinese shoppers outdid themselves, buying $9bn (£5.8bn) worth of products on the country's biggest shopping platform, Taobao.

'Fighting time'

Chinese web retailers need little excuse to set up new shopping events. Right after Singles' Day, huge discounts were offered on Black Friday, originally an American shopping event, and 12-12, on 12 December, another numerically pleasing date.

Image caption The delivery operations now have to run 24 hours a day in order to keep up with the huge demand

Christmas Day is a shopping holiday in China too.

So, it's no surprise the courier industry is expanding. Almost all of the orders logged on Taobao are sent through independent couriers, like ZTO, the company that employs Mr Yang.

The chief executive of ZTO, Lan Bushi, looks exhausted in the wake of Singles' Day. He's only been sleeping four or five hours a night since China's peak online shopping period began.

"On a normal day, we handle seven million packages," he explains. "Suddenly, it's gone up to more than 20 million a day."

"We're operating 24 hours a day non-stop. People change shifts but the machine keeps running. We're fighting time," he adds.

'Just enough money'

Delivery trucks leave the courier hub and fan out across the city.

Individual couriers like Mr Yang hand-deliver about 100 packages a day.

Each package drop-off requires a customer's signature, stretching out the delivery process.

Recipients slowly amble over to Mr Yang to half-heartedly scratch their initials on his delivery form. He fidgets as he waits for them, desperate to move on so he can deliver as many packages as possible in a day.

Image caption Individual couriers like Mr Yang can hand deliver about 100 packages on a normal working day

"I make just enough money to provide food for my family," he shrugs. He has no choice but to hurry.

In addition to his wife and mother living in his rural village, Mr Yang has two daughters - one is 20 and one is five. When he mentions the five-year-old, his face breaks into a big smile.

But there is little time for chatter. As soon as the last package in his sack is handed over, he rushes out of the building, and over to his delivery bicycle to retrieve the next round of packages.

No time to stop while China's eager online shoppers are at their computers, placing new orders.