When considering training - or many other pursuits - it is common to consider what do you get out for the effort you put in. Basically, is it worthwhile. The inputs and outputs can be tangible and intangible, emotional and physical, financial or not. Whatever...there's a lot of input and output things to consider.
In the context of training, more is more means doing more training to get more fitness / performance / enjoyment / etc out. It sounds simple...do more to get more. What could possibly go wrong. Well, a lot!!!
More is more is good advice depending on context. If you're starting from zero, doing more than zero training will get you more than zero fitness benefit. However, this should be followed carefully to avoid the age-old problem of doing too much too soon!! Nevertheless, for many people simply doing a bit more training - which can be achieved in a range of different ways - will usually bring about more fitness. Just be careful how much more of what part of your training you do, and how quickly you increase it. (Ask a coach to help advise you!)
But for people who are already at a good level of fitness from a good workload of training, doing more to expect more can end in disaster. You might get some short-term improvement, but you might also run into longer-term problems with any of injury, fatigue, burn-out, and other things. Doing more is always appealing, and looks good on Strava(!!), but is loaded with risk. Remember, when things are going well, be conservative. (Ask a coach!!)