Change the subpanel to a much larger panel
The reason the wires won't fit on the subpanel is that you shortchanged yourself and bought a panel that is only 100A. Possibly you thought this was required. If aiming to save money, the Polaris connectors and pigtails will cost more than the price difference to a bigger panel.
The othe problem with 100A panels is they are small in terms of number of spaces. 100A is a lot of power (and you actually have more still), and can power a lot of stuff. Some of those things take 2 breaker spaces. Many of them are not allowed to use "twin/duplex" double-stuff breakers, because almost every 120V circuit these days needs AFCI and/or GFCI, and they don't make those breakers in double-stuff. You really don't want to be backed into a corner of having to cram more circuits into the too-small panel.
So tear that thing off the wall and take it back and get a subpanel with enough spaces, and most likely a 150, 200 or 225A main breaker. The wires will fit just fine on that.
Change the feed breaker to a larger breaker
Look at the wire you bought. Look at Table 310.15(B)16, in the 75C column for your wire... and read the amps listed. Now, round up to the next available breaker size. That is the largest breaker you are allowed to use on this wire.
Buy a larger feed breaker, up to that size, that will fit your wires. If you bought 2/0 wire for instance, that is rated for 135A, round up to 140? 150? - regardless, it should fit just fine on a 125A breaker.
"But wait, I didn't follow your above advice and my subpanel is still 100A" -->> Okay, does the subpanel have a 100A main breaker? Yes, then you're still fine, because that will protect the subpanel from overload.
Don't confuse "bump for voltage drop" with "maximum amps"
The whole thing you do with calculating voltage drop, the 3% and all that jazz, is optional. You can run 5, 6, 7% voltage drop if you're accounting for ALL the voltage drop that will occur from the meter to the ultimate load, and keeping that sane e.g. 8%.
This is why you are freely allowed to breaker a long wire run right up to the max allowed for that wire (e.g. 150A for 2/0). Wire amp rating doesn't care about wire length.
What about the ground? When you upsize wire for voltage drop, you're also required to upsize ground in proportion -- that better not be a #8 ground. As a result, the ground is good for whatever the hots are good for.
Pretend you ran 2/0 wire, and intend to pull 100A actual of load.
- Because you enlarged the wire, at 100A actual, your voltage drop will be exactly where you want it to be.
- Because the wire is safe to 150A (albeit at worse drop), you can breaker at 150A.
- Because you plan for 100A actual of load, you must provision 125% of that, or 125A in both wire capacity and breaker. This requirement is satisfied.
- You can use a 200A subpanel. Obviously, the 150A feed breaker protects the 200A subpanel with good margin.
- The 200A main breaker in this panel is unlikely to ever trip, but it serves as a shutoff switch.