Symonds said aerospace-derived technology helped Mercedes build an ultra-efficient cooling system that enabled it to create the narrow side box layout unveiled on the first day of the Bahrain test.
Symonds, who was the main architect of the 2022 regulations, admitted he had not anticipated such a dramatic interpretation of the rules.
"It's a whole new approach," he said in an interview with F1 TV's Ted Kravitz.
'I love seeing new interpretations'. I have to say it's not what I was expecting. And I'm still very impressed with the way they let the air through to cool the car, but they obviously are."
"And I think that will have caused their rivals to go back to the rule book with their red pen and see exactly what they did."
Symonds admitted that when his engineering team wrote the rules and created a full-size model of the 2022 car, they anticipated the need for larger radiator intakes.
"I think it was just a little more radical than we thought," he said.
"When we were developing the aerodynamics of these regulations, of course we looked at a lot of things, not just generating downforce, but we had to look at brake cooling, we had to look at tire heating, and most importantly we had to look at look at engine cooling.
"And we've used a bigger entrance than this [on the W13] to get cooling down. I think at the Mercedes they have a few little tricks to help them with this.
"So for example the intercooler is a very, very neat device, it's a water-to-air intercooler, which of course Mercedes has had for some time, but I think it's a little bit different.
"And that's why they can really shrink this car a bit more than most of the others."
George Russell, Mercedes W13
Photo by: Steven Tee / Motorsport Shots
Expanding on what Mercedes had done, he added: "The intercooler I was talking about I think came from Reaction Engines in Oxfordshire, the people who make this sort of air-breathing rocket engine, and the sort of offshoot of which this really was extremely efficient heat exchanger technology.
"And I think that's one of the reasons they were able to produce the car the way they did."
Symonds pointed out that all the teams have perfected their packaging to make the side boxes as small as possible.
"I think one of the trends we're seeing, and it's not specific to these new regulations, but we're finding that it's very, very difficult to put everything into side modules.
"People think that what's in the side modules is surely just the radiators, the heat exchangers?
"But of course there's much more, there's a lot of electronics in it. I think some people are moving these electronics to this keel area. Aside from the side boxes. Nothing else on the W13 caught Symonds eye from the cooling concept.
"I think the rest looks, dare I say, reasonably conventional, if there is such a thing as conventional. The treatment of the front wing, exactly what we had expected.