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In recent years phone books haven't made as much of a thud on doorsteps. In fact, they seem quite small. Are these once hefty books getting thinner?

Phone books have long been used as more than just a tool for finding people. They've served as doorstops, booster seats and packaging filler.

Ripping the thick books in half used to be a classic trial for a strongman.

But, this year, more than 27 million people across the UK may notice that the books appear to be thinner. Why?

Each phone book now covers smaller geographic areas, according to BT.

A single directory was once produced for London. Now, there are six directories for inner London and 20 for outer London.

Across the UK, BT prints 168 directories, while Yell, the company that produces the Yellow Pages, makes 104 different directories.

The dimensions of both the BT phone book and the Yellow Pages have shrunk.

While the height of the new BT directories remain the same, the width was reduced by 31mm (1.2in) to 172mm (6.8in) earlier this year.

The Yellow Pages changed the height of their latest phone book by 5cm (1.97in) and the width by 4cm (1.57in), after research showed a change in size would increase overall usage by 12%.

Both books are now slim enough to fit through standard size household letterboxes, avoiding the once common sight of unwanted books mouldering on porches.

Could the shrinkage be because of a switch from landlines to mobile phones, which are not in the book?

Research conducted by telecom watchdog Ofcom suggested 14% of households in the UK had mobile phone access but were without a landline into their home in 2010, up 2% from 2009.

Since most people do not publish their mobile numbers and others elect not to share their landline number, could the phone book be facing an imminent demise?

Despite the drop in the proportion of people using landlines, the growing population means the number of listings in the phone book is stagnant rather than in sharp decline.

The content in the phone book has been rearranged, not lost, says a spokesman from BT.

The typeface in both the BT phone books and the Yellow Pages have been altered to fit more information on smaller pages.

These changes come partly in response to criticism from environmental groups that producing and delivering large phone books each year is wasteful, BT says.

BT's revamped directory - which is 15% smaller - is printed on recycled paper and will save about 2,000 tonnes of paper each year, BT says.

The new Yellow Pages - which is a mixture of paper either recycled or sourced from sustainably managed forests - will conserve about 5,000 tonnes of paper a year.

Together, the saving is equivalent to slightly more than three times the total weight of the wheel and capsules of the London Eye.

Still, with people able to opt out of both being in and receiving a phone book, many question whether or not they need one.

Both BT and Yell insist their directories are still widely used.

In a recent survey sponsored by BT, eight out of 10 households had an up-to-date copy of the phone book and 91% of people still used it on a regular basis.

The Yellow Pages are used 19 times a second, says a spokesman for Yell.