There is no direct relationship between the physical size and performance.
The form factor defines the size of the plastic shell. What is inside is based on the same chip manufacturing technology. Technology improvements, in the industry overall or by a specific chip manufacturer, can go into either form factor.
Putting a MicroSD card in an SD adapter doesn't change it's characteristics.
The adapter doesn't perform any signal processing, it is basically just a spacer with electrical contacts.
Thanks to Jason C for the picture from camerahacker.com
Form Factor Differences
The MicroSD form factor has a lower maximum capacity and lacks a write-protect switch. The smaller format is generally more expensive at the current time. This Wikipedia link describes the technical differences in the specs for the two sizes (and many other memory card formats).
There are different classes of performance, and cards can be manufactured in any class, regardless of size. However, at any given time, there may be less availability of super-fast cards in one or the other format, and there may be specific examples of cards in one format that are faster than supposedly similar cards in the other format.
Manufacturer Speed Designations
Even within the same manufacturer, the labeling doesn't necessarily tell you anything about the inherent differences between the form factors. For example, SanDisk could have developed a faster card in a MicroSD format than their last fastest card in the SD format, and labeled them both "Extreme". That doesn't mean the MicroSD format is inherently better. It just means that right now, you can buy a faster MicroSD card from SanDisk than their SD card with the same label.
Nature of the Cards and Manufacturing Process
Don't think of these cards as identical, precision parts. It is the opposite.
There is great variability in the manufactured output, and the process deals with yields and salvage. The product labeling is based on what bucket post-production testing puts them in.
The raw product is a commodity, and the quality and quantity of what is available in the marketplace varies. One comment describes a certain memory card seller as using "catch-of-the day".
So the performance of a specific MicroSD card may be less similar to an "identical" card from the same manufacturer as to a specific SD card from another manufacturer.
The Bottom Line
You can't rely on the form factor or the manufacturers' model names to know what is currently the fastest available card. The form factor is not the driving variable and the manufacturers' product names aren't specific or reliable measures of performance.