You might not need to buy the second-generation large HomePod to get additional smart home features — they could already be present in the speaker you have. MacRumors notes that Apple's HomePod product comparison section (scroll to the bottom) now says the HomePod mini has a temperature and humidity sensor, just like its more expensive counterpart. Given that the sensor has been dormant since the mini launched in 2020, it appears that Apple is activating the functionality on the more affordable hardware. We've asked Apple if it can provide more details.
The temperature and humidity sensor lets you control smart home devices based on interior conditions. You can automatically close your smart blinds when it gets too hot, for instance. The HomePod mini is already known to be getting the sound recognition update that can detect smoke and carbon monoxide alarms and send notifications. That software arrives in the spring.
There are still reasons to buy the bigger HomePod besides raw speaker quality. You don't get room sensing or spatial audio with the HomePod mini. For everything else, though, the mini should do the trick. The question is whether or not any expanded functionality is enough to clinch a purchase. Amazon Echo speakers have had temperature sensors for years, and they can also play Apple Music. Google's Nest Audio doesn't have the sensor tech, but it should sound better. The mini makes the most sense if you like its tighter integration with the Apple ecosystem.