Thus, a direct comparison is desired, for example, "The thesis is at the about the same level as an average Tekn.Dr. thesis in Sweden".
This is also fine: "The thesis is below average compared to a typical Tekn.Dr. thesis in Sweden, but would still meet the required standards for a Tekn.Dr. degree".
On the other hand, we do not want an "adjusted" comparison of the form: "However, considering the relatively shorter time spend on the thesis (3 years versus 5 years in Germany) the thesis is at the same level as the average German doctoral thesis."
On the other hand, this statement is acceptable (because this is a direct comparison, with an additional comment about how much time the student used): "The thesis would rank somewhat below average in my country. However, the student has only used 3 years to complete the work, which is short compared to the average in my country" (of course, this assumes that the student actually has used 3 years - which is rather rare - and that the 3 years do not refer to the nominal time to complete a dr.ing. degree.).
Please note that, although the nominal time to complete the dr.ing. degree is 3 years, the average completion time is more than 4.5 years.
The important issue is that the dr.ing. degree should satisfy international standards for a doctoral degree in the given subject area (about the same as a Ph.D. from a good US University). The statement from the foreign member of the committee is to help us to maintain these standards.