This might be a weird question, but I was wondering what it would be like if the opposite to “cram” lamps existed. For example, would it decrease effectiveness of light output if a 100w mercury vapor lamp was placed in a BT56 envelope? What about a 25w incandescent in a PS30? I know that it would probably increase the manufacturing cost for lamps, and not be very practical.

Verz likely it would just increase cost and reduce the practicality of use...

They technically do exist but for decorative lamps. You can get decorative incandescent and LED filament lamps in lowish lumen packages 800L but in large envelopes like BT56 or PS52. These are sized normally used for 1kW lamps. 

I believe I did see a lower wattage CMH in a one-size-up envelope. I believe it was either a 70W or 100W ceramic MH in an ED-28 envelope, because it was meant to be a replacement for 175W mercury vapour (or MH). The idea was that it would keep the luminaire as is, with the arc tube properly centred in the reflector, as well as the mogul lamp holder. You only needed to change the ballast and the lamp instead of fiddling with fitting a medium base holder or adjusting a mogul holder with an adaptor. I also believe it had a rather short retail period. There is a picture somewhere here of one.

See here.

I believe I did see a lower wattage CMH in a one-size-up envelope. I believe it was either a 70W or 100W ceramic MH in an ED-28 envelope, because it was meant to be a replacement for 175W mercury vapour (or MH). The idea was that it would keep the luminaire as is, with the arc tube properly centred in the reflector, as well as the mogul lamp holder. You only needed to change the ballast and the lamp instead of fiddling with fitting a medium base holder or adjusting a mogul holder with an adaptor. I also believe it had a rather short retail period. There is a picture somewhere here of one.

I believe those typically used a ED23.5 envelope. I have a special hot restrike Philips 100w CMH that's in the larger ED28 envelope to allow for the higher voltages from a hot restike ignitor.

Some of those Derby 100 lamps but with a 120v filament and a E26 base would be really cool, I have a Holophane incandescent wallpack that's sized for 150w incandescent so using 100w lamps would have the filament too high up in the reflector and the socket is fixed.

This lamp seems to be the opposite of a "cram" lamp:

https://www.ebay.com/itm/Philips-C50S68-High-Pressure-Sodium-Lamp-Light-Bulb-50W-BIG-MOGUL-BASE/162483764486?epid=1500624063&hash=item25d4c97d06:g:a9IAAOSw5UZY-TXP

Very big envelope and mogul base for just 50 watts.