Well, in my opinion there's quite a few different angles to this question.
1: Is this a system which has been designed to minimum requirements, but not to actual loads.
2: Is the overload affected on only a few branch circuits, or a certain subfeed.
3: Does this effectively overload the Transformer[s] feeding the occupancy [>130% FLKVA].
4: Does this overload any SDS [Separately Derived Systems] in the occupancy.
5: Was this something that the designing Engineer was aware of prior to the project's completion.
Virgil's message with the Minimum service size per NEC Art 220 [Computed Loads] I think covers it very well.
Is it just as simple as figuring the service size per a calculated load? In most 1 family dwellings, it may. In a Commercial Warehouse, it easilly could fall too low!
We built many Tilt-Up Shells with 400 amp 208Y120 VAC services and I could see this being exceeded very easilly for T.I.'s if the Tenant has high machine usage.
Even in "Strip Malls", the NEC Minimum load calcs can be exceeded. Case - In - Point:
A particular Client of mine who owned and operated Tanning Salons.
At one location, the Salon occupied two separate units, divided by the usual Dimising Wall. Each unit had an existing 100 amp 208Y120 VAC 4 wire service, feeding into 125 amp 24 space panels.
After figuring all the proper calcs for the Tanning Beds [multiple types], the HVAC, Lighting and all that jazz, Both services and panels were upgraded to 200 amp. This just barely made the calcs!
Although there was never any overloading of the main OCPDs, the calcs just made it!
I would imagine that if any more loads were added, plus all were active coincidentally [at the same time], the main, or mains would become excessively heated - very close to tripping the OCPD!
If the systems were loaded up high, plus the other occupancies were at high loads, the Transformer would easilly fail! [75 KVA for 10+ services].
For at least 3 years after I did that project [and before the location was sold to someone with very low maintenance skills], I never heard of any overload problems.
This approach in my words was pure "Crapshooting", since the potential for overloading several key items was present, but never occurred.
Simply to sum it up and answer the question, IMHO if the system has been designed with the installed / design loads in mind [as per outlet locations and branch circuit considerations], but the entire COMPUTED LOAD per service[s] / subfeed[s] / Transformer[s] are allowed to remain inadequate, that's an overloaded system to me.
The last Bank Branch I Engineered required no less than 200 amp 208Y120 VAC service, just to conform to extreme basic NEC calculations. If I had used those values when designing the project, there would have been a serious overloading of the main OCPD!
I opted for a new 400 amp service, which cost the Client $20,000 more for the total Electrical Installation [on a $1.5 Million project]. There was absolutely no reluctance from the Client! I gave them all the pros and cons to consider, and they still had no problem with the extra upfront money.
It would have costed upto 20x the amount if done at a later time.
This is the approach taken when I do any system's design - for any EE project.
Sometimes you just can't allow for every situation [such as the Tanning Salon], other times you can [such as the new Bank Branches].
Just my opinion.
Scott SET
