Steve added this to the tech description in the database last year.

Commercial Jump Drives are larger, less efficient and cannot be used by ships with military engines

There is a more detailed explanation in the v5.1 suggestions thread.

OK, so the jump capacity is : min(Jump Ship Size, Jump Drive Capacity). Thanks for the tip, I did not understand that point when reading the tutorials.

Speaking about tutorials, the wiki says :

Ships with commercial engines can be escorted by ships with either military or commercial jump drives

vs.

• Commercial jumpdrives only support commercial ships and the same restriction is on military jumpdrives.

Can you confirm one or the other version ? (I'll correct the wiki if needed, it's a useful tool for beginners 

My kneejerk response is that a simple forum search would have answered your question.

These are common mistakes that seem to be reported by new players every few weeks: 

• The jumpdrive being used must be large enough to cover the jumpship itself.
• The size of the jumpship is maximum limit to ship size that can jump with it not the limit of the drive.
• Commercial jumpdrives only support commercial ships and the same restriction is on military jumpdrives.
• Squadron transit is limited to the task group that the jumpship is a part of and the squadron size limit includes the jumpship.

 

2 Jump Tenders are waiting at a JP :

Ticonderoga class Jump Tender    35700 tons     506 Crew   [...] 
JC49K Commercial Jump Drive     Max Ship Size 49500 tons    Distance 25k km     Squadron Size 2

Forrestal class Jump Ship    4150 tons     388 Crew     [...]
J6750(3-50) Military Jump Drive     Max Ship Size 6750 tons    Distance 50k km     Squadron Size 3

I send a small military ship to this location with the 'standard transit' order. Here is the pretender :

Belknap class Corvette    4850 tons     463 Crew     401 BP  [...]

And the message I get in the Events Log :

26th November 2053 05:34:31,Sol,CT Belknap 001 cannot carry out its standard transit order as at least one ship is larger than the ship with the highest jump rating. No friendly or allied jump ships are available for escort

Considering the relative size and jump capacities of the ships, I don't understand why the jump fails. 

NB : Squadron jump also fails, of course.