GBG can be a bit of a joke when one guild has 76 members blasting away at sectors while they compete against guilds with 7-8-9 members. Kind of makes it a ridiculous contest. May as well just award them the win and be done with it.
One of my worlds has a mid platinum Guild of 2 players. I see there is a one (1!) member Guild in mid Platinum in your world with 120 more MMR then your Guild. I doubt they spend much time worrying about being up against Guilds 5. 10, 40 times their size.
It's not all about the size of the dog in the fight, it's also about the size of the fight in the dog.
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You might as well get used to the consequences of your Guild's successes. When you take first like last BG, you will always get get a huge bump in MMR. This time it pushed you into low Platinum and you're facing Guilds that may have
1 - Lost badly in their mid or upper Platinum previous BG.
2 - Belong in low Platinum or high Gold
3 - Won big in their mid Gold previous BG.
1 - means that Guild could have lost up to 175 MMR the BG before 3 - means that Guild could have won up to 175 MMR the BG before.
1 - and 3 - happening in the same BG means Guilds can be matched up that had differences of 350 MMR the previous BG.
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There's a lot of factors that can contribute to wild swings like that.
The previous BG record of your oppostion. Are they playing above or below their norm?
Guild membership changes, gaining or losing even a few good GBGers in a moderately active large Guild can make a huge difference in performance, causing rapid changes in MMR. More on that a bit later.
Guilds might intentionally tank a BG as a form of R&R, causing a large loss of MMR but setting up their next BG against much weaker opponents.
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One of my Guilds was middling Gold when I joined. About the same time 3 other active GBGers also joined. We finished 1 and 3 in the next two GBs gaining 250 MMR putting us in mid Platinum At which point 2 of those players left the Guild. Our next competition was against a Guild that was in Diamond the BG before but had lost out putting them in mid Platinum.
We're getting smothered. We'll drop back into mid or high Gold for a while until we can train up or recruit some more good GBGers.
Size. Quality. Activity.
You've chosen to keep your Guild much the same size since you first complained about being matched up against bigger Guilds. Even though you've got quality members your Guild will keep getting torn up by bigger Guilds that have more quality active players. When you get matched against them.
As I said before, it's not all about the size of the dog in the fight, it's also about the size of the fight in the dog.
But having the biggest meanest dog with the most fight in it is bestest.
Meanwhile on topic:
Two Guilds now in Diamond through wildly different means. One is crammed full of bloodthirsty fighters who don't deal with anyone, the other uses diplomacy to achieve success. The Platinum Guild is referenced above.