The main problem is that your cronjobs is not working right, and because of this some tables are not pruning and grow a lot !
sys_banners_shows - 1.8 GiB
sys_sessions - 422.1 MiB
bx_spy_data - 262.5 MiB
bx_spy_friends_data - 74.2 MiB
sys_sbs_queue - 65.6 MiB
bx_photos_views_track - 135.7 MiB
bx_videos_views_track - 34.6 MiB
sys_profile_views_track - 22.2 MiB

I suggest to truncate all above tables manually (spy data will be lost but it will be collected again very soon, it will not affect see more any critical functionality) and make sure that cron job is running correctly.

I suggest to run pruning process when your server is not loaded, to change pruning process start time change `time` field in `sys_cron_jobs` table in the following record (clean /cache/ folder after this change):
cmd 0 0 * * * BxDolCronCmd inc/classes/BxDolCronCmd.php

Also I suggest to increase the following settings in your MySQL server: tmp_table_size and max_heap_table_size
set it to 256Mb to 1Gb depending of free RAM on the server

Additionally your server can be speed-up a lot, please refer to these tutorials:
http://www.boonex.com/trac/dolphin/wiki/HostingServerSetupRecommendations (especially webserver section)
http://www.boonex.com/trac/dolphin/wiki/TutorialCachingTypes
http://www.boonex.com/trac/dolphin/wiki/TutorialUnderstandingCacheEngines