Here are some spec's for the BigDog cited on Wikipedia.
_Dimensions: 2.5 ft tall, 3 ft long
_Weight: 240 lbs
_Engine Size: 15 HP go-kart engine
_Computer: PC/104 stack, Pentium 4, QNX real time operating system 
_Speed: 8 mph
_Carrying Capacity: 340 lbs
Size: One of the main reasons to do order-of-magnitude estimates (OME's) is to get a feel for the size of a design problem before you start. But doing OME's requires a good engineer's intuition to start with. The BigDog is a great working example of a robot that fits the profile of a robot harvester/tender. As such, it offers a great test case to exercise one's OME intuition skills.
In physics and engineering, every problem brings with it its own size, mass and response-time scales. For the case of a robot harvester/tender, it has to be bigger than the plants it will be working on, yet it will have to be light enough to be "wrangled" without the use of extra equipment to pick it up and/or move it around. This indicates something the size and weight of a motorcycle; that is, dimensions of 3-5 ft and 300-400 lbs or less.
So, if the BigDog can be used as a good example, then it appears that our attempt at an OME has proven valid.
Engine Size: Here is where the design spec's for a robot harvester/tender will differ from those of the BigDog.
The BigDog, by the nature of the tasks it's designed to do, will experience high peak loads with large swings between low and high continuous loads. This calls for a power source that is both "throttle-able" and has a reasonable power response over a wide range of loads. The engine that fits this requirement is a piston-driven ICE.
On the other hand, a harvester/tender ‘bot only needs to move along at a steady speed of 2-20 ft/min. The tender ‘bot, only needs to carry is its own weight. While, the harvester ‘bot, will need to carry the extra weight of the produce it’s harvested and the weight of any packing boxes it will need. Either way, the total work load for a harvester/tender will be lower and much more even than that of the BigDog. In which case, one can sacrifice the requirements for "throttle-ablity" and wide power-band for a power source with a much greater efficiency than a piston-driven ICE.
Next Post: Piston vs. Turbine Engines.
 I would recommend that people check out their website and look at the various robots they build. Boston Dynamics has its own YouTube Channel , too.
 This is actually not a very large computer core by embedded-systems standards.