As the branches diverge, merges take more and more time to do (up to a couple hours, at which point we abandoned the model)... they won't be done automatically. Since merges are basically context-free it's hard to determine the "logic" of changed lines. Since merges always contain a bunch of changes, all have to be resolved before they can be tested, and tracing failures back to specific conflict resolutions takes extra time. Reviewing a merge is seriously difficult. Mismerges are also far more likely to go unnoticed in a large merge compared to a smaller cherry pick. With cherry picking you are only considering one change, and you know which one. You only have to resolve that one change, and can then test, if you feel that's necessary, or move on to the next change.
When maintaining multiple release lines and moving fixes between them:
Don't use a bad branching model. Things like "merging upwards" (=committing fixes to the oldest branch requiring the fix, then merging the oldest branch into the next older branch etc.), which seems to be somewhat popular, just don't scale, don't work very well, and produce near-unreadable histories. They also incentivise developing on maintenance branches (ick).
Instead, don't do merges between branches. Everything goes into master/dev, except stuff that really doesn't need to go there (e.g. a fix that only affects a specific branch(es)). Then cherry pick them into the maintenance branches.
RES isn't physical memory, either. I can happily have, say, 30 GB RES on a system with only 1 GB RAM (and no swap).
smem's measures - USS, unqiue set size (unshared pages) and PSS, proportional set size (shared pages divided by their share count, ie. how many tasks share them), are far more accurate and easier to reason about than RSS and VSS.