Tvheadend

Depends on a few factors, such as whether you are encoding the file from the source to disk or you are just passing it as a raw transport stream. The other factors are around how the broadcaster is sending the data, the bitrates they are using and how the multiplex is formed. Sky UK and Freeview use statistically multiplexing on their transponders which means that bandwidth is shared between all available channels with each taking more if required (at the detriment of other channels sometimes). This can mean that if there are channels that are not broadcasting (or are broadcasting low bandwidth or static content) that other channels get allocated high amounts of bandwidth.

I see such behaviour with the CBeebies channel in the UK - the SD variant often broadcasts at over 10Mbps which is 3 times as much as an SD channel really needs.

If you tune to a channel then switch to the Status > Subscriptions tab and look for the channel you are streaming then check the 'Input' and 'Output' bandwidth values - if you are just passing the stream with no transcoding/encoding then these values will be identical. This basically tells you how much bandwidth the channel is using in kbps - you can then divide it by 1000 to get Mbps and then use this site to convert the bandwidth to an estimated disk usage value. It's not an exact science (due to the bandwidth being variable) but a rough guide would be;

3 Mbps    0.375 MB/s
4 Mbps    0.5 MB/s
5 Mbps    0.625 MB/s
6 Mbps    0.75 MB/s
7 Mbps    0.875 MB/s
8 Mbps    1 MB/s
9 Mbps    1.125 MB/s
10 Mbps    1.25 MB/s
11 Mbps    1.375 MB/s
12 Mbps    1.5 MB/s
13 Mbps    1.625 MB/s
14 Mbps    1.75 MB/s
15 Mbps    1.875 MB/s

The usage you are seeing would mean a bitrate of about 22Mbps which is very high but not uncommon given the variable nature of DVB encoding (especially if it's a HD channel).

Thanks for the explanation. Yes it is an HD channel and set for pass through. It is an OTA channel in the US. I changed it to matroska so we'll see how the size comparison works out.