as you know, a map file is basically an archive that contains all the tags. often times the same tag is complied into multiple maps. Wouldn't it have been easier and better for bungie to instead just direct the map file to a central tag directory instead of compliing each tag into every map? map files would be a fifth if not less of the normal size..

does anyone get what im saying, and/or agree with me?

It does that.
Bitmaps.map

Halo 2 and halo 3 do this as well in shared.map mainmenu.map and campaign.map

i mean like instead of putting the tags into, for example, bloodgulch.map, point the bloodgulch map TO the tags directory so the tag data isnt in the map file itself.

Yes I get what you are saying and other games such as UT and Source games do just that however there are downsides for that methodology. When I worked with the UT1 engine many map makers would make changes to the core textures files instead of creating new ones so once you installed the new map the old maps would not work properly.

Halo PC/CE does use some core tags that reside in the bitmap.map file but not as much as some other games. There are pros and cons to both systems that have to be considered. For example the H2V maps rely on shared maps files (shared.map and single_player_shared.map) for its maps and the map maker is severely limited because of it. The H2V maps are much smaller in size but there is less you can make changes to. That is the tradeoff between map size and map flexibility.

ohh ok.

We have he tag set for custom content, and building of .maps. But that's a privilege we get from Bungie, usually, a game would not include the source files, just a compilment of data. Your logic makes sense, but remember that the tag set is from the SDK, not Halo CE.