Size matters. Especially when we’re talking about software. Quite simply, smaller is better.
In some cases, size comes at a cost. We call this a lossy compression. JPEG is such an example. The smaller your JPEG, the lower its quality. There is no way to recover the original data. It is lost. The loss may be acceptable – it may not even be visible to the naked eye.
In other cases, compression is lossless. PNG and ZIP are such examples. When you compress something to a ZIP file, you expect to be able to recover it in its entirety when you unzip it. A PNG preserves the original details of an image so that the two can be rendered identically. These compressions are usually not as efficient or not as performant. That is, compression or decompression takes a lot of computing resources.
So why does all this matter? It matters because many users have limitations. Their bandwidth may be limited. Their storage space may be limited. It would be a shame to lose these users.
Other ways to lose those extra kilobytes: for photos, prefer JPEG. For clipart, PNG may be better. Experiment. Try lowering the quality of your JPEGs. Try downsizing them and scaling them back up on screen.
A slim app is more attractive. Size matters.