=Technics used=
===Mode1 and screen dimensions===
The use of Mode1 enabled the CPC to keeps the same graphics as Spectrum, in appearance only.
While the square pixels looks almost the same, the code was completely different BTW.
Also CPC being set for a 320x200 display instead of a 256x192 display, the pixels are in fact smaller on screen, hence a bigger border...
Biased argue is to say that the CPC resolution is therefore inferior to spectrum's one, because there is a large border on CPC while ZX Spectrum displays a smaller border.
The amount of pixels is exactly the same, and this looks even more detailed and finer on CPC.
While some Spanish companies reduced the screen in order to get the game betterly running in low-specs 464... the infamous speccyports just ran a reduced screen so the ZX Spectrum's codes and graphics could run almost "unchanged".
===Real-Time Convertion of 1bpp graphics===
Here is the real grief. A common way to get the Speccy game running on a CPC was to get some routine to convert the existing and already coded 1bpp GraphicDatas Speccy's Graphic Datas (Tiles and sprites ... in equivalent to Mode2 on CPC) into 2bpp datas (proper Mode1) when put into the video RAM.
Needless to say, this enabled to use no additionnal graph artist (a costly man at the time) to redrawn the Graphic Datas...