The guest output cache will cache unique views, so if you're getting more guests looking at more unique pages, the amount cached will increase. This cache is periodically truncated by a task (use Cron to ensure it's cleaned up often)
You can disable output caching with a constant OUTPUT_CACHE_METHOD, set to 'None'. - https://invisioncommunity.com/4guides/advanced-options/configuration-options/using-constantsphp-r25/
You may see slower loading guest pages after changing this.