I'd say it's very likely that something else is making the difference, and you're misattributing it all to weight.

As noted above, for a given speed up a hill, the weight penalty is creating maybe a ~1% difference in required torque. That's only one quarter of the difference between a 25-tooth big cog and a 26-tooth big cog, definitely not something that'll stand out that much.

If the issue is that you're bottoming out your gearing and being forced to stand, the first thing I'd check is that you're appropriately-geared for the riding that you're doing. "Appropriately geared" means that you're able to keep your torque and cadence reasonable per what your legs want, not that you simply didn't have to get off and walk. Plenty of people insist "my gearing is fine" as they climb at 40rpm, even when it's costing them rapid fatigue and a 15% loss in wattage.

What chainrings and cassette are your SC using? What chainrings and cassette did you use on the old bike?