I see more and more static methods and classes last 2 years maybe. It's probably more about the stateless design and less side effects, but it definitely helps garbage collection if you avoid classes at session scope or smaller. In OOP there is another pattern that helps - object pools, but it's a lot of work to get it to work correctly and it's not as efficient.
If most of your classes are small, I don't see why people have to resort to static methods.
If your methods are static, there are tendency/lust to use static member variables (hence stateful) which will cause side effects.
Don't forget the following points too:
1) You still have pooled objects somewhere (stateless business logic classes like XYZServices, repository classes that may be backed by pooled DB connections and Transaction Managers) provided/managed by your Application Server or by 3rd-party framework (Spring does this).
2) Your Application Server tend to have beefy hardware, good enough not to care of GC hiccups.
There are other reasons to use static methods but I don't think they're strong enough in this case.
Small classes are actually worse GC-wise. Because they will fill up the GC graph with many small nodes as opposed to fewer large nodes, which are released in bulk with little fragmentation. Small and large nodes have the same GC overhead essentially. In general you want your objects to be large. When they are small, once the GC realizes what's going on (usually at some high threshold 90% or so), it will have to run a some O(n^x) graph reduction algorithm or defragmenataion. Special tuning is required for such cases. Beefy hardware doesn't help in many cases due to locks. There are very few production-ready lock-less GCs.
Object lifetime is a much more important factor than class size for most server request / response style processing.
Typically there are three lifetimes for objects in server processes. Those that are allocated around startup and are never deallocated; those that are allocated per-request and become garbage once the response goes out; and lifetimes that span multiple requests, like objects in caches.
The first are normally ultra-cheap to "collect": with a generational GC, you simply don't scan them at all, because they haven't changed.
The second group, per-request, are also fairly cheap to collect. Every so often, you GC the youngest generation, and you only need to keep track of references in registers and on the stack. Ideally many requests will have occurred between collections, and the only objects that get kept alive are objects that are in-flight for the current request. And this is why you need at least three generations; you really don't want to have to scan the oldest generation to collect these ephemeral objects after they've built up over a number of youngest generation collections.
It's the third group that kills you. You can save on the cost of scanning the whole heap, using write barriers to track new roots buried in the oldest generation; but that adds accounting costs, and eventually overtakes the cost of a whole heap GC. These guys can also cause the fragmentation you're worried about - they need to be compacted down, copied possibly multiple times. On the CLR, last time I checked, you need a full gen2 GC in order to get rid of them, as they've likely survived a gen1 collection.
With these guys, it's worthwhile doing the big object thing. In fact, it may be worthwhile not having any GC heap storage for them at all, and refer to them using different techniques, like ephemeral keys that look up in Redis, or native pointers stored in statically allocated arrays.
In app servers I've designed, I've never seen GC CPU usage over 5% or so, even with heavy usage of tiny short-lived objects. But you need to care about lifetime.
We're talking in the context of stateless Request <-> Response of the Web-Application nature here.
When a Request comes in, the App-Server will allocate (or use from the pool) a thread to serve that Request (in .NET/JVM world, Ruby/Python uses Processes unless you use different App Server).
If you create small objects within the scope of that Request (which usually lives inside a method) and that objects are contained and don't hold references to any long-lived objects, they will be GC-ed quickly (and potentially way quicker) once the method is finished.
Thread is GC-ed as well once it's finished (unless you wish to release them back to the 'unused' pool).
My feeling is that their use of static methods have nothing to do at all with GC.
