I can’t say for sure it’s all BfA zones, but at minimum the WM shard comprises the BfA continent you are on (edit: nope I think it’s just the zone, but still.). So that’s quite a large scope for player distribution.
A % of those players will be leveling, so they are just spread out. Another % are wq-ing in general. A % are at specific wqs like the azerite and emissary ones. A % are WPvPing. A % are AFK. You can see the potential for spreadage, and then we have an assault which naturally leads to clumping in that subzone during its course. And LfG is another factor. Lots of potential for a fluctuating population and distribution - with a base shard faction balance.
Sometimes it’s hard to form a counter raid. That’s just part of WM. Battlegrounds are for fixed numbers in a subzone. WM is not a battleground. It’s chaos, many factors affect difficulty - a challenge!
The easiest way to retaliate, is to form your own group, then you have control over the group title - which makes or breaks your group forming. For example, name your group “4 Bounties Tiragarde Assault” - you should get a raid going, unless a competing raid in LfG.
Takes patience. Members will leave as other join - but when you hit a critical mass of raid members, maybe 15+, then members tend to stay in raid. Not always easy, not supposed to be. Tactics like avoiding enemy raid until you have enough in your raid helps. As well as LfG, use local defence to recruit, nearby allies, your guild, community, friends.
If there’s 100+ Horde at an assault in your shard, there could be 100+ Alliance in another shard. And every other shard with random distribution. 100+ enemy at an assault will just be a lag fest though, and I never saw that. If outnumbered, I usually see more like 30+ Horde, which doesn’t lag me. But at every assault it can be different, sometimes outnumbered, sometimes outnumberer. Assaults are long as well, will fluctuate over its duration.
If you join a group, you should normally leave that shard. Luck of the draw to get a better enemy/ally number. It’s all part of the fun though.
Consider joining a WPvP guild and /or community. Fun to do assaults with players you know.