Actually we have some fairly massive muscles such as our abs, traps, and calves which are made up mostly of slow twitch fibers. Now with that established i think i can answer your question. You asked if big muscles are always piles of fast twitch fibers. Well most muscles are, but like i mentioned those 3 muscle groups are not. Now the way that we train these muscles is by doing a lot more reps. For instance the abs and calves are muscles which are usually trained to failure, the reason being is that in order to make them bigger or stronger we must go after the slow twitch fibers which require more reps. The traps too are muscles which require more reps, meaning if say somebody usually does only 1 to 5 reps per set, like in a case where he is training for strength, he will still do somewhere close to 15 reps for his traps. So what im saying is, yes we do workout those muscles differently, we do more reps. By doing this we will not lose muscle as you pointed out in your question because that only happens if your endurance training, but if the muscles is made up mostly of slow twitch fibers then doing more reps is not endurance training .