Hello Ricardo,
the right question is whay my router has built a so big table?
check if you have for example a default route towards internet link using interface name instead of an ip address next-hop.
if so your device is using proxy-arp for every internet destination and this is not efficient.
the change in this case is to configure a default static router with an ip address next-hop that of provider router.
Another possibility is that there is an attack by a connected device sending a lot of gratuitous ARPs to your device.
Hope to help
Giuseppe
