It’s a restriction in PHP, not WordPress. The best solution is if you have access to php.ini, make the following changes:
memory_limit = 32M
upload_max_filesize = 10M
post_max_size = 20M
This, of course, depends on your hosting company. Most don’t allow direct access to php.ini, but offer alternative solutions to change just those values you need to alter. You’d have to check out your hosting company’s policy.
You can also try to add this info into the .htaccess file by adding:
php_value upload_max_filesize 10M
php_value post_max_size 20M
php_value memory_limit 32M
But most hosting companies will restrict users from using .htaccess to modify php.ini information. Check with your host on the best solution, but these are the only two options.