the long skinny can is about 10 inches long and the short fat is about 7.5 inches long, the inside capacity of each can is almost the same measured after they were built.
I know what you mean by the pipe running inside the cans but doing a search for MCM or mousse can muffler builds, brings up about 95% empty can designs and to tell you the truth, I haven't actually seen a picture with the pipe inside the can like the Jett and similar tuned muffler except for the stock muffler modification mousse can shown on spad to the bone website. Just very few posts that describe the pipe any more than 1/2 - 3/4 inch inside the can.
I am going on the logic that bigger volume containers can use smaller exhaust stinger and smaller containers need larger diameter stingers. I did read on some sites that if the can is too large of a volume for the engine then it will not "get on pipe" this was from some very knowledgeable profilers judging by the amount of engines they have experimented with.
I did play with the idea of making a similar setup to the Jett mufflers or ultra thrust mufflers but it just seemed like the straight header pipe length with the included radius on my header would be too long overall with the size of can they have on their mufflers. I could probably do it with a sharp 90 bend which is what they kind of have on theirs.