Relative Size is a Priority of Choice

Olympus OM-D E-M1 with Sigma 50-500mm Four Thirds lens next to the Sony A7 with 35mm f/2.8 Samyang pancake lens

People who have not gravitated to the Micro Four Thirds or Mirrorless ecosystems often take the example of a large MFT body and large MFT lens to demonstrate that the smaller sensor of the MFT system is pointless when the large MFT combo can be / is larger than a smaller-by-design full frame system. I realised that indeed, I do have the requisite gear to demonstrate this aberration and made this photo, impromptu.

The trouble with big trouble with waving big hands and making statements that seem obvious like that is, the truth is often other than obvious.

This specific Micro Four Thirds gear

The E-M1 and Bigma combo looks monstrous. It is. And not the most comfortable or effective for Birds In Flight - in fact it's far from optimum in most use cases. The E-M1 is not the smallest MFT body - it's made weather resistant, tougher, with a big hand grip to offer ergonomic comfort when mounted with larger MFT Pro Level Zooms. It and subsequent flagship Pro Level Bodies is packed with mechanical and electronic features - fast frame rates, sophisticated autofocus systems, the most effective in-body stabilisation and so on. So, it is not a good demonstration of a small light body for travel and street photography.

The Sigma 50-500 zoom is a huge zoom from the era of Four Thirds E-System DSLRs when the E-3 body was not particularly small and lenses for it were not well optimised for super small size. Lenses in those days were not much smaller than the competing APS-C or Full Frame DSLR lenses. In fact, it is likely that this Bigma is not specifically designed for a Four Thirds sensor and could be an APS-C (or full frame) model grafted onto a Four Thirds lens mount.

I bought the Bigma for old times sake - to play with a now discounted lens that I had heard so much about from the old days.

This specific full frame example

On the other hand, the Sony A7 (original version) was specifically made as small as possible for a full frame model. Sony chose lens mount called the E-Mount, originally first shown on the Sony NEX APS-C models. New competitors in the full frame mirrorless market are pointing out in a mischievous manner that their lens mounts, designed fresh, come with larger orifices to potentially enable f/0.65 lenses.

In this initial A7 model, Sony could not figure out how to include in-body image stabilisation into the system nor silence the strident sound and of the full frame shutter.  In this and in subsequent models, the hand grip is particularly truncated in contrast to the Nikon or Canon DSLR grips. There is no AF joystick, the buttons are not backlit for night work like the DSLRs.

The Samyang 35mm lens is particularly compact. It is only f/2.8, the autofocus is slightly noisy and the bokeh is not the smoothest.  I chose to buy it because I wanted something small, light and less expensive.

"Real serious" lenses that full frame owners (for example, those owners of the Nikon D850 and Canon 1D / 5D) prefer are typically 1.4 (at least), have a substantial number of glass elements to correct for all sorts of aberrations, ensure smooth bokeh, ensure 48 megapixel resolution and sharpness. Such no limits lenses are in no way small, light or inexpensive.

Meaning

Although the MFT sensor is a smaller cropped sensor, you don't have to design or choose gear that is small. Conversely, although the full frame sensor is larger and gear tends to be larger, you can opt to sacrifice design targets to make specific gear smaller.

Food for thought