"U" in IaaS is Universal.
IaaS (either one) can be universally appropriate, whether you are a small business or mega corporation.
First let's talk Infrastructure as a Service. Typically this refers to Cloud providers, like IBM SmartCloud Enterprise or ProfitBricks or Amazon and many others. The Infrastructure they provide is processing power for rent. This concept is not new, in the old days (yes I was there then) not many could afford mainframe computers so you rented time on those computers called time-sharing. The pendulum swings and time-sharing is back in the form of IaaS of virtual computing assets.
Small businesses can now afford to timeshare a virtual computer on someone else's data center with internet bandwidth ten or even a hundred times greater than they might even be able to get in their business and if they could they couldn't afford it on their own. The IaaS vendors can offer significant processing power and bandwidth for a fraction of the conventional costs, e.g. ProfitBricks pricing for a 4 Core/16 CPU 4 GB Ram and 100 GB HD with Linux OS is just 142 Euros/mo, IBM is only slightly more.
Now let's talk Integration as a Service. While this too is not new, it is growing and is now beginning to be offered alongside other Services. Typically Integration as a Service is the ability to rent an Integration Server with an Enterprise Service Bus running on it and also typically running in the Cloud. Mulesoft has their own CloudHub where you can run your Mule ESB applications to integrate applications and resources anywhere.
There are a few vendors that offer both forms of IaaS alongside SaaS and PaaS, like @O4BO