737800s dont generally tanker above 5 tonnes when temperatures are below 10c, this is to avoid wing ice forming as a result of cold 'soaked' fuel.
By avoiding one need of deicing the likelyhood of a deicing delay is avoided. Wouldnt fancy a trying a ryanair 25minute turnaround with a deice crammed in there!
However if a standard environmental deice/antice is anticipated then tankering larger quantities may occur.