Take a look at the License Overview Dashboard in the BigFix Console, you’ll see the three components that are shared between Lifecycle and Inventory:BES Asset Discovery, BES Inventory and License, and Maas360 Mobile Device Management.
Theoretically, you could use the licenses in the manner you’re describing, but you could not use the features and functions of BigFix Lifecycle like Patch, Software Distribution, OS Deployment, Server Automation, on the 25K machines with only a BigFix Inventory license. This approach doesn’t make much sense to me. Instead, it seems likely that you have 25K machines, with each machine being licensed for both Inventory and Lifecycle.
While BigFix Inventory is not shown as a separate Domain within the Console, and is instead subordinated to the BigFix Lifecycle Domain, it is still licensed as it’s own product within the BigFix family.