Why could a SQL Server table grow far quicker than the data to it?

I've got an audit table in SQL Server 2008 R2 that has the following structure:

CREATE TABLE [AuditInstance](
    [oid] [int] NOT NULL,
    [AuditTransactionOID] [int] NULL,
    [pid] [int] NOT NULL,
    [tid] [int] NOT NULL,
    [ObjectKey] [nvarchar](20) NULL,
    [ObjectType] [nvarchar](40) NULL,
    [ObjectName] [nvarchar](255) NULL,
    [Operation] [nvarchar](20) NULL,
    [AuditData] [nvarchar](max) NULL,
PRIMARY KEY CLUSTERED ([oid] ASC)

About 3 weeks ago it had 106,168K rows and a data size of 115GB (reserved size 121GB). Today it has 106,917K rows (less than 750K more) and consumes 287GB (with 294GB reserved). Each of those values are taken from SQL Server's "Disk Usage by Top Tables" report and so does not include the non-clustered indexes which have barely grown in this time. I calculate that the average growth per row for that period to be 230kB but the average size of the AuditData field for the same period is:

select avg(datalength(AuditData))/1024.0 as avgSizeInkB from AuditInstance 
    where oid < 107006115 and oid > 106006115

avgSizeInkB
---------------------------------------
0.530273

Even if all of the the other fields were full to capacity (which they're not) this should only add another 662 bytes per row for a total of about 1.2kB. Can anyone explain why the table seems to be growing almost 200 times quicker than this? (PAD_INDEX is off and no FILLFACTOR is set so I don't believe it relates to that, although I'd assume that would show in the reserved size rather than data size anyway).

Further info:

The results of:

SELECT type_desc, total_pages, used_pages,data_pages 
FROM sys.allocation_units
WHERE container_id in (SELECT partition_id FROM sys.partitions 
WHERE OBJECT_ID = OBJECT_ID('AuditInstance'))

type_desc           total_pages   used_pages    data_pages
IN_ROW_DATA         185863        185806        185109
IN_ROW_DATA         8826738       8824615       8801687
LOB_DATA            27101311      27098965      0
ROW_OVERFLOW_DATA   0             0             0
IN_ROW_DATA         583149        581879        575613

suggests that it's mainly LOB_DATA.