This is wrong and a often repeated myth.
1. Tanks fought infantry and emplacements more often than tanks in WW2, so higher caliber = better since it deals more damage.
2. The 122mm gun of the IS2 could penetrate slightly more mm of steel than the Panther.
3. The gun of the IS2 and also the kwk 36 8.8 of the tiger could penetrate any regular tank with ease despite less velocity than the 7.6cm of the Panther. So in most situations higher caliber = more damage = better.
Ehm no.
1) get an 88/75mm he and tell me if it isn't enough to kill you.
The military doctrine that survived today is the use of high velocity guns for tanks, while artillery, still powerful he, siege guns, hmg's could all deal efficiently enough back in ww2 and to the point it is not worth sacrificing at performances.
Ww2 is full of tank vs tank engagements.
2) FACTUALLY WRONG
https://en.m.wikipedia.org/wiki/122_mm_gun_M1931/37_(A-19)
Max penetration at 500ms: 155mm
https://en.m.wikipedia.org/wiki/7.5_cm_KwK_42
Max penetration at 500ms: 234mm
Even using pzgr 39/42 with lower velocity kwk42 got higher penetration: 168mm
3) that's not that simple
3.1) firing a lower caliber shell is much less expensive.
3.2) the gun of is2 couldn't penetrate king tiger at any but close range, nor panther beyond 1500ms, king tiger could penetrate is2 at an extreme long range, panther could penetrate is2 under 1500ms, so it was an even fight, but if is2 missed, it was 20 seconds reload, and was pretty much doomed.