The company, which is aimed at 16-24 year-olds in particular, said its main womenswear range of dresses and jackets, as well as its new menswear website, also saw strong sales.
Pre-tax profits jumped 129% to Â£14.3m in the six months to August.
It also benefited from successful marketing campaigns, especially on social media.
"We have... stepped up our activity on social media on Facebook Live and on Snapchat and a content drive on Instagram has helped us pass the two-million follower mark in the UK," the company said in its statement.
This has been a large part of the company's success, analysts said.
While it is still much smaller than rival Asos, Boohoo has been particularly savvy about "communicating with its core 16-24 year-old customers," said retail analyst Kate Ormrod from research firm Verdict.
It uses bloggers and "influencers" to showcase its designs.
"As the customers mature, keeping them interested will be a challenge," Ms Ormrod added.
Asos has 12 million active customers, while Boohoo, which started in 2006, now has 4.5 million active shoppers.
The fashion industry as a whole as been trying to attract the lucrative plus-size customers, said Ms Ormrod, and that's something that Boohoo has done well.
"They haven't always been offered the most fashionable styles and it's a strongly growing market."
The joint chief executives, Mahmud Kamani and Carole Kane, said there was "robust growth across all regions and continued momentum in new customer growth".
Revenue rose 38% in the UK, 41% in Europe and 93% in the US.
It plans to introduce a new range of children's clothing, as well as a small maternity range.
Strong sales of plus-size and petite clothes have helped boost profit at online fashion retailer Boohoo.