Hi,
Do you have thumbnails in your categories ? Do you have same problems in product listing ?
I know that image resizing could use a lot of memory (because the image has be uncompress in the memory to be resized).
With a special configuration, it is possible that HikaShop should regenerate all thumbnails and, if there are big images, it could use a lot of memory (even if we "unset" the image data).
It could be an explanation, but there are a lot of other possibilities (in HikaShop, in other plugins or in Joomla).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.