Think of it this way, part of the paradox was that it grew more so the farther back in time it went, the other part is that it even existed at all, as it's like the chicken and the egg. If it starts in the future, it cannot grow backwards in the past since it wouldn't have existed yet, but the fact that it not only exists, but is larger in the past would suggest that it did exist before the future timeline, thus the paradox.
Also, remember that the tachyon pulses were found to be sustaining the anomaly, and that according to this logic, the future timeline actually created the problem with the anomaly with the tachyon pulses, which is why it was bigger in the past. It may never have happened at all, and might just have been a random, harmless phenomena had the tachyon pulses not been fed into it. It took all three timeline Enterprises to go into the anomaly and create static warp shells to collapse it. The collapse of the anomaly caused the ships cores to be breach, but once it collapsed it didn't matter, as the result of what the future timeline had done to cause the problem in the first place had been reversed, causing the past and present to be restored.