Diskspace usage of DB

Question

I have a production database that's consuming a significant amount more disk space than it should be.

Using sp_spaceused (only the interesting tables), I get.

Table Name    Reserved (KBs)    Data (KBs)    Index Size (KBs)
----------    --------------    ----------    ----------------
E              149600             142992         1136
H              184968             12104        162272
M               94104             29512         62480
T               25616             13544          9464
MH              16072             10640          5208
U             5399312           2984184       2414376
Total         5869672           3192976       2654936

However, the on disk size is a whopping 16 GB database and 6 GB log file.

The culprit is mainly Table U. Most of the data is very static with just lots of UPDATE to existing rows. Table U is the ony with loads of data being added constantly, and being archived weekly.

Table U's definition is,

[ID]      INT                         NOT NULL,
[RD]      DATETIME                    NOT NULL,
[UN]      CHAR (7)                    NOT NULL,
[A]       BIT                         NOT NULL,
[C]       BIT                         NOT NULL,
[S]       DATETIME                    NOT NULL,
[rowguid] UNIQUEIDENTIFIER ROWGUIDCOL NOT NULL

It receives about 5 million rows per week, and archives another 5 million rows per week.

It has not been partitioned / placed into separate file groups.

The system has been arround for approximatley 6 months, and I don't believe our DBA team have ever shrunk it / defragged it!

Questions

1. Why is SQL using so much more disk space than sp_spaceused says?
2. Is the churn responsible for most of this disk space issue?
3. How should I use (SQL commands please) partitions and file groups to better optimise this churn?