I was bored and interested, so I figured this out.
Of 60 Soviet vehicles, excluding derps, lower calibre weapons are the best choice on:
T-26* - 37mm over 45mm.
T-46* - 45mm over 76mm.
T-28* - 57mm over 76mm.
BT-2* - 37mm over 45mm.
BT-7* - 37mm over 45mm.
A-20* - 45mm or 37mm over 76mm.
T-34 - 57mm over the 76mm.
A-43 - As above.
T-44 - 100mm over 122mm.
AT-1 - Debatably, the 57mm over the 76mm.
SU-76 - Again, debatably the 57mm over the 76mm.
SU-101 - 100mm over 122mm.
Of 41 British vehicles, excluding derps, lower calibre weapons are better on:
Valentine - 6pdr over 75mm.
Of 71 German vehicles, excluding derps, lower calibre weapons are better on:
Pz1C - 8mm over 20mm.
Luchs - 30mm over 50mm.
Leopard - As above.
VK 20.01 D - 50mm over 75mm.
VK 30.02 D - Arguably the L/70 over the short 88, but it's very dependant on playstyle.
DW2 - 50mm over 75mm.
Pz IV A - 50mm over 75mm.
VK 30.01 P - As with the 30.02 D, arguably the L/70 over the short 88, but dependant on playstyle.
JgPz IV - One could argue for the L/70 over the short 88, but they're probably wrong.
Rhm-Borsig - 128mm over 150mm.
WtPz IV - 128mm over 150mm.
WtE 100 - 128mm over 150mm.
Should be noted that on the Borsig and the WtPz IV the 150mm is a viable weapon, but the 128mm is definitely the first choice.
Of 17 Chinese vehicles, excluding derps, lower calibre weapons are better on:
Vickers B* - 40mm Pom Pom over 45mm.
Chi-Ha* - 47mm over 57mm.
Type T-34* - 57mm over 76mm.
T-34-2 - 100mm over 122mm.
110* - 100mm over 122mm.
Of 13 Japanese vehicles, excluding derps, lower calibre weapons are better on:
Type 97 Chi-Ha* - 47mm over 57mm.
Type 1 Chi-He - Possibly the 57mm over the 75mm, they're both okay-ish guns and I haven't used the tank.
Of 65 American vehicles, excluding derps, lower calibre weapons are better on:
M7 - 6pdr over 75mm.
M8A1 - A case can be argued for the 57mm over the 3-inch(76mm) gun, but it is not my choice.
Of 37 French vehicles, excluding derps, lower calibre weapons are better on:
Renault FT* - 25mm over 37mm
ARL 44 - 90mm over 105mm, however, the 105mm is still a popular and viable choice.
AMX M4 45 - As above.
S35 CA - 17 pdr(76mm) over either the 90mm or the 105mm was my choice. However, they are all equally popular, if not the 105mm in the lead.
ARL V39 - A case can be argued for the 90mm over the 105mm, but I much prefer the 105mm on this tank.
Vehicles marked with an asterisk denote where the higher calibre weapon is a lower tier weapon, and thus more obviously a lesser/ downright retarded choice.