Is there a proper word for software that is enormous for what it does?

On origin: "bloat" arrives from the ordinary sense of swelling — something distended beyond its natural size, usually with gas, and usually unhealthily. It is a deliberately unflattering metaphor and always has been.

It attached to software in the era when a program's size was a thing you felt constantly, because you were counting floppy disks and later worrying whether something fitted on a CD. When a text editor grew past what seemed reasonable, "bloated" was the natural word.

There is a related coinage worth knowing, because it names the mechanism rather than the result: feature creep, sometimes creeping featurism. That describes how it happens — each feature is individually justified, nobody removes anything, and the sum is enormous. Bloat is the state; feature creep is the process that produced it.

The pair covers your question fairly completely: feature creep causes bloat, and Wirth's law describes the race it wins.

The established term for the phenomenon is software bloat, and for the process specifically there is a better and older name: Wirth's law — software gets slower faster than hardware gets faster.

Your instinct about "bloatware" is right and it is a genuine split in usage. It started as your sense — software bloated relative to its function — and was then largely captured by the preinstalled-junk meaning, to the point where most people now hear only that. So it is not wrong, it is ambiguous, and in a sentence about an application you chose to install it will be misread.

For a single program, the phrase that survives without ambiguity is just bloated, used as an adjective: "the app is bloated". Nobody misunderstands that, and it does not carry the preinstalled implication.

There are two more that are narrower and useful if you want to be precise about the cause rather than the symptom.

Dependency bloat — the program is enormous because it pulls in a large tree of libraries, most of which it barely uses. This is the one behind most modern cases and it is why an application that does very little can still ship hundreds of megabytes.

The second-system effect, from Brooks — the tendency for the follow-up to a successful simple thing to be overloaded with everything that was left out of the first one. It names a specific and very recognisable failure, and it is often the honest diagnosis when a version 2 is four times the size of a version 1 that people liked.

If you are writing about a specific program, naming the cause lands harder than "bloated", which people read as taste rather than analysis.

One caution about using any of these in writing about a specific product: the words carry an accusation of carelessness, and quite often the size has a boring reason — bundled runtime, multiple architectures in one package, debug symbols nobody stripped, an installer carrying assets for twelve languages.

None of that is bloat in the Wirth's-law sense and the word will be argued with. "Ships a 400MB installer because it bundles a whole browser runtime" is both more accurate and harder to dismiss than "bloated", and it survives the reply from someone who knows why.