Sitting on a road saddle is a learned skill, and requires a fit that is decent. I got a saddle computer pressure fitting x2 and found all that out.
So too narrow if you are missing (vs hitting) your “sitz” bones, and tend to fall off one side or the other. Simply not enough platform to perch on. This is where a Brooks or hard saddle is great because in no uncertain terms if you fit is off you’ll know it!
Too much pressure down the middle or pressure to one side, not a feeling of being secure on the saddle - that’s how I would describe it.
You can’t be too wide for short durations or upright comfort riding - ie a Brooks flyer for example. The only downside to too wide is chafing or discomfort inboard on the hip joint/leg area (i have read). I prefer just wide enough myself, but can’t ride too small or narrow!