ADS-B does not require a transponder, though transponders may have ADS-B Out functionality.
ADS-B Out uses a GPS receiver (these currently fit inside of cell phones) a computer (one fits inside the cell phone along with the GPS receiver) and a transmitter capable of 250W (or so, about the same as a low end car sound system) with an average power consumption under 10W. It may be convenient to have an external GPS antenna - a few centimeters on a side and maybe a centimeter thick. Of course, the ADS-B antenna. The requirement for barometric sensing is met with a chip that is often included in cell phones to get a better estimate of altitude than GPS can provide, particularly in estimating position inside of buildings. A typical such sensor is the MPL3115A2, offered in a 5 mm x 3 mm x 1.1 mm package.
A functional ADS-B In system can fit in a shirt pocket, complete with SDR (synthetic digital radio) receiver (many are the size of a USB memory stick) the typically used Raspberry Pi computer, the display and a small speaker as well as the battery to power it.
It is unlikely there is insufficient room for the functionality to fit, but it is possible certain makers are uncaring about the amount of space they use. What may be a problem is for ADS-B In - there needs to be room for the display, depending on how large a screen one wants or needs. They also make a larger than likely required input screen and keypad for setting some ADS-B Out parameters. Example:
https://www.mypilotstore.com/MyPilotStore/sep/14316 is a functional replacement for an aircraft that already has a transponder, integrating the ADS-B into the transponder package.
Just to emphasize - ADS-B is a beacon system, not a transponder system. ADS-B Out is an information source operated like a navigation lamp without regard to any response triggering input.
It can be packaged as shown here:
https://costaero.com/product/skybeac...ions=SkyBeacon
No affiliation or endorsement of any seller or manufacturer - these sites are from what Google suggested for "ads-b system"