Reduced carriages causing overcrowding

Hi - I commute daily on the Waterloo to Salisbury line and am getting increasingly frustrated at overcrowding as a result of reduced carriage numbers.  Both yesterday evening and this morning, I travelled on trains each missing three carriages, meaning I and many other passengers had to stand for 1 hour +.  This seems to be happening more and more regularly - multiple times each week.  until the last few months the situation was very rare - I can only think of a single occasion in 2.5 years commuting under the previous franchise holder that this happened.  Can someone explain why this is happening so regularly now?  Is it the result of a change in operating policy and thus set to continue, or is it a result of short term issues with rolling stock?  In either case it is extremely unpleasant - far worse than a 15 minute delay, for which I can at least claim compensation.