Prohibitive size of random forest when saved to disk
The size of each tree depends very much on its depth. Thus, change the maximal depth (max_depth). Try to set it to finite number (as opposed to the default "None") and then try to reduce this number. In addition (or as alternative) try to increase min_samples_split or min_samples_split.

You can also analyze you features and keep only important ones. The simplest way would be to have a look at the clf.feature_importances_ of your forest. (In general, finding important features is an art and science on itself.) Exclude non-relevant features and rebuild the forest.