We already know by now that the global Xperia Z1 Compact (D5503) came to the market with a back made out of a high-rigid plastic sheet. This was confirmed by Sony after much confusion, where at least initially the company made claims that the handset came with a glass back.
The confusion lay due to the fact that the Xperia Z1 had a glass back and it was believed that the Japanese version of the Xperia Z1 Compact, the Xperia Z1 f (SO-02F), did too. This much has been proven in the following video which compares the rear parts of the Xperia Z1 Compact and the Xperia Z1 f.
Under the hammer, the Xperia Z1 f’s rear shatters due to the use of durable tempered glass, whilst no matter how much the Xperia Z1 Compact is flexed, no shattering occurs. The use of glass in the Xperia Z1 f is also the reason why the rear is heavier at 12.2g versus the Xperia Z1 Compact at 8.2g. The use of glass in the Xperia Z1 f is also the reason why Sony used an anti-shatter film (ASF), which is not found in the Xperia Z1 Compact.
Rear part: Xperia Z1 f (left) versus Xperia Z1 Compact (right)