Undivided XP - a potentially unbalancing idea

 I was inspired to make my home game a West Marches campaign from a play-by-post game I've been in for a few years. It's a 5E game, so it has a different scale of progression anyway, but since it's PbP, the DM doesn't divide XP among the group members. If we face a group of monsters worth 700xp total, we each get 700xp. 

Since PbP is a really slow moving way to play, this means we still level up fairly often, especially with 5E's expedited numbers for leveling. 

In my game, the groups recently tend to be on the large size, so much so that most of the players have stopped hiring men-at-arms. They tend to be around level 4 to 5, but with a few at 6 and a few still at 3. But despite the presence of level 5 and 6 PCs, they still tend to think they're only able to handle the level 3 stuff. Fair enough, it has been a deadly game (no dead PCs this past weekend, but two weeks ago one PC and the last of the henchmen were killed). 

I'm wondering if maybe I should switch to a thing like the PbP DM does, and not divide the XP. Being that this is Classic D&D, I'd of course stick to the rules that no more than one level can be gained per session, so when they get a dragon or giant's massive treasure, we won't see someone shooting from level 3 to level 7 at once. It would speed up advancement which would help the lower level PCs catch up level-wise, and maybe give the players a bit more of a feeling of toughness. 

Part of me feels like they're operating below their capacity for risk, sticking to the safer areas, and avoiding some of the dungeons (which often are a bit more challenging than the area they're in). 

Part of me feels like I should just let them do this, as it means I can take my time on expanding the keyed areas of the map. If they're leveling faster, I'll need to prepare stuff in farther regions faster than I am right now. 

Also, part of me feels like this will unbalance things. The benefit of a huge party (6 to 8 PCs, a tiger and a dire wolf as mounts for two of the PCs, plus occasional henchmen or leveled retainers) is that they can handle more danger. The drawback is that the XP gets divided more ways. 

I probably will not implement this idea, but it is an interesting idea to consider. Maybe in another campaign some day.