After the Xperia Z Ultra's launch event in both London and Shanghai, we had a brief chance to talk to Sony's Product Design Director, Jun Katsunuma, who was present in the latter city. Jun's been responsible for Sony's mobile devices since the Xperia S days, so the transition to the Xperia Z's double-glass design was also under his watch. That said, the newer Xperia Z Ultra isn't simply just an enlarged version of its smaller sibling, as we found out straight from the horse's mouth.

"The frame [on the Xperia Z] is made of plastic, then we picked a PMMA (a transparent thermoplastic) for the plate," Jun reminded us while holding both devices. "But this time we use aluminum in the side plates, which means it's totally different, then we could realize this super thinness [on the Xperia Z Ultra]."

On a similar note, Jun pointed out that the Xperia Tablet Z is more like the Xperia Z, except for the former's reinforced fiber back. Even then, the tablet is thicker than the Xperia Z Ultra by 0.4mm, courtesy of the latter's aluminum contruction.

Here's another subtle difference that Jun shared with us. While the 6.5mm-thick Xperia Z Ultra has the same black, white and purple options as the thicker Xperia Z, the designer intentionally chose not to give the new phone the same glittery purple paint. The reason being a solid purple apparently just looks better under the larger piece of glass. And it's not Gorilla Glass here, by the way, but Jun and his colleagues assured us that the glass on the new phone is just as strong -- especially since the super responsive touchscreen (this is likely either Atmel's or Synaptics' solution) supports pen and pencil input.

Sony's Jun Katsunuma on the inspiration for Xperia Z Ultra's design

Without naming the elephant in the room, Sony Mobile's Marketing Programme Manager Benoit Obadia acknowledged that there's indeed a stronger request for bigger displays on phones these days. But as to why his team specifically chose a 6.4-inch screen size, Jun said it was simply a matter of finding the largest panel that he could fit into the width plus a similar thickness of a passport -- something that travelers are used to carrying around all the time, especially in their coat pocket. Similarly, seasoned travelers are also likely to be carrying a pen, which can be used for scribbling on the Xperia Z Ultra. That way they won't have to carry a dedicated stylus.

"The point of this Xperia Z Ultra is how we could let the users bring this one into their travel and daily life," said Jun. "That's why we chose aluminium to realize our product's proposition."

Lastly, we pointed out the potential fatigue over the all-too-familiar design ID of the latest Xperia-branded devices, but Jun and Obadia implied that at the moment they're not too concerned due to the positive response. Still, the designer realizes there needs to be a balance, and his team solves this problem by implementing some subtle but practical differentiation.

"At Sony, we need to bring some of that message with consistency," said Jun. "The Xperia Z, the Xperia Tablet Z and the Xperia Z Ultra are very strong in consistency, but of course, they have different propositions. So that's how we decide to use some different materials, different pigmentation and sometimes construction. Based on this product and based on our one design language, we really do some differentiation but with high consistency.

"The use case of the smartphone is changing, so we need to consider how we can express the differentiation in any use case in any product. So we create our designs based on this high-level concept."