As Boss Hoss said, weighing brass only tried you for much gravity it has; nothing to do with capacity.
If you want to know something, measure it.
Here's a perfect example, from pressure testing I was doing on a Creedmoor with LRP Vs. SRP.
Average weight - 165.1gr
Largest spread - 0.5gr
Average weight - 154.78gr
Largest spread - 2.2gr
Same powder charge from the same lot, same BTO with the same bullets from the same box. The extremely minor differences recorded by the Pressure Trace, were low end of statistical noise.
A very real possibility for why the change, is the different alloys used. Lapua claims to use the original WWII Olin C260 alloy, with an undisclosed heat treat spec. Federal brass on the other hand, is notably softer; sometimes dangerously close to the case head. They do this for faster obturation, which almost always improves accuracy/burning curves. The flip side of that coin, is case life isn't always so great for a reloader. And if one is simply praying to the alter of "signs", it's a further reason why staring contests don't reliably inform actual pressures.