If you are a developer, regardless if it is on Android, iOS or Windows Phone, there are guidelines. Those guidelines tell developers how things are to function in their app, how things are to be laid out and how big things are suppose to be in it. The idea is to provide a uniform user experience regardless of the app the user happens to be in at the time. Google, when it comes to Android, is no different but when it comes to the guidelines around icon sizes in the launcher, developers haven’t always adhered to the rules. So Google is forcing the issue. In the latest update to the Google Now launcher and the Google Search app, icon sizes are uniform thanks to a change in the software the reduces the size of larger-than-they-should-be icons.
With the update, which is rolling out now to the Google Play Store, regardless of the size of the icon that the developer has created for their app, the Google Now launch will enforce a size limit on it, shrinking it down to the guideline sizes per the design guide. If it sounds draconian, it is a little bit but to be fair Google isn’t doing anything that Apple or Microsoft doesn’t do already. For their part, Apple will simply reject your app if your icon is larger than it should be and the same holds true for Microsoft. Google has been a bit more lax on it to this point but it would seem those days are over.
So why as an end user would you care? Aesthetics mostly but also by having icons a uniform size, it lowers the chance of you accidentally “side touching” an icon when you meant to open the app next to it.