I was recently informed that there is such a thing as a 400 watt BT-28, and a 1000 watt BT-37. I didn't know these existed, but it's pretty cool to see such a big arc tube crammed into a small bulb. Take a look.

Have a look at the tubular T15 lamps, the arc tube can sometimes be as close as 1/4" from the outer envelope. I have a tubular Ushio 400w halide thats near EOL and the outer envelope has developed a significant raised bubble (about 1/2"). The arc tube has a raised bump as well

That's why they are sometimes referred to as 'cram lamps'.

cram lamps never last as long as the full sized ones..and using a cram lamp in a full size lamp fixture makes the light distribution look funny

I don't care for the BT-37 1000W MH or the reduced envelope 400W MH. Their life is not as good as the full-sized versions. I work as an electrician for a local mall. They have fixtures that use both versions of the 1000W MH lamps, some 65' in the air. A few poles had heads recently replaced as part of a new store contract. The new heads are top-loader LSI that require the BT37 where the original Cooper are bottom-loader using the large enveloped lamps.

The heat build-up from the BT-37's causes the capacitors to fail at 1/3 of the hours of the larger lamps. The cooper fixtures are dated to '98-99 where the older LSI's are from 2002. 75% of the '02 caps have been replaced where 50% of the older caps have been replaced. Same brand, model cap in all the fixtures. Hours on the pole heads averages 7-9 hours per night, depending on location in the lot and its assigned function in the energy management system.

I too deal with a lot of MH fixtures, and the trend is currently very low profile, compact fixtures with compact ("cram" mogul or medium screw, or less frequently tubular or double ended) lamps, these small fixtures are also beginning to use more and more electronic ballasts housed inside the fixture within inches of the lamp, and im not talking the 39w-70w CMH, the 250w-400w sizes are also now using electronic ballasts resulting in high failure rate

yep I've seen it too..especially the cooper 400W MH shoebox fixtures they are just too small for that high of wattage and capacitors seem to always fail under 10 years they don't even dim out much just eat lamps and then fail completely, rarely do I change fried ballasts
the 250W version I've never had to change a ballast on yet!