I suppose everyone has at least heard of the slangy terms handjob & blowjob, both relating to sexual activities. However, the other day I was watching the TV show Friends and I heard the term boob job referring to getting breast implants. I wondered if the same would apply to a woman who'd like to get implants on her buttocks, perhaps buttjob, or something along those lines—— but all I could find was more sexual insinuations about that term as well. Don't get me wrong, I don't have my mind in the gutter here, but why is it that the term boob job seems to have no sexual insinuation if it's formed the same way as the other three? I understand that handjob & blowjob wouldn't convey the idea of getting implants in one's body, but at least buttjob or assjob could work the same way boob job does.
There are many job phrases, especially in slang, and while there are blowjobs and handjobs and rim jobs, most jobs do not have sexual connotations. They simply follow one of the various conventional definitions of job as a task, a piece of work, an occupation, or a crime. Referring to such things as jobs is informal; in formal registers, I would only use job in its basic sense of an occupation or duty, or when part of an idiomatic set phrase like odd job or joe job.
There are mcjobs and day jobs for the under-employed and the under-discovered, lube jobs and salvage jobs for pre- and post-crashed cars, and hatchet jobs and snow jobs for your enemies. If you pull a job on a bank you've done a bank job, which might make you a nut job or a whack job, especially if it isn't an inside job you thought up at your desk job.
Its use to refer to cosmetic surgery is so common that the American Heritage Dictionary includes it as a definition:
An operation done to improve one's appearance, or the result of such an operation. Often used in combination: a face job.
In addition to nose jobs, there are eye jobs and, as you note, boob jobs, but not every type of cosmetic surgery is best known as a job: there are various lifts, tucks, and implants, for example, for more specific work. It is common to hear breast implants instead of a boob job, being somewhat more formal but not as clinical as breast augmentation, and being more descriptive of the procedure, as technically speaking, breast reduction and breast shaping could also be argued to be "boob jobs." And rather than a butt job, you're more likely to hear of butt implants as one method of buttock augmentation.