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It is not just humans that are steadily growing in girth, webpages are going the same way too.

The average page is now about 965 kilobytes in size, reveals a study of top sites by the HTTP Archive.

The figure is 33% up on the same period in 2010 when the average webpage was a svelte 726 kilobytes.

Keeping webpages small is likely to become more important as increasing numbers of people browse the web on the move.

Analysis suggests the bloat is down to user demands for more interactivity, as well as the tools used to watch what happens when people visit a site.

To gather its figures, the HTTP Archive ran a series of tests every month on the web's top 1,000 sites.

These showed that average webpage sizes were trending steadily upward throughout 2011 and jumped sharply in October. Big pages generally take longer to load, which can mean visitors quit if a page takes too long to appear.

The metrics the HTTP Archive gathered suggest some causes for the growth. Images are a big proportion of the average webpage, and the higher resolutions people expect have led these to grow.

However, the statistics reveal that the category showing the biggest growth is that for Javascript. This scripting language is widely used to make webpages more interactive and responsive.

in the amount of Javascript on webpage was down to the growing use of HTML5. This is the latest version of the formatting language that defines how webpages should be written.

The growth could also be down to the use of web analytic scripts that tick away unseen when someone visits a page and which log what they click on.

Freelance web developer Anna Debenham said large pages could take too long to load, leaving people frustrated and likely to go elsewhere.

There were many different tricks that developers could use to shrink the size of a webpage and ensure it loaded quickly, she told the ´óÏó´«Ã½.

"There's always room for some optimisation," she said, but added that because many developers were self-taught, few were well-educated about how to slim page sizes.

However, she said, the need to ensure webpages were pared to the minimum would become acute as more and more people browsed the web via mobile devices.

"We've had a bit of a lull where we all thought everyone was using fast broadband," she said. "But now we are almost back to where we were with dial-up because mobile speeds are so slow."