As computer game technology has increased, graphics have become higher in resolution. Terms like 'chunky' and 'blocky' have often been used to describe computer graphics using larger pixel sizes, either in comparison to contemporary graphics of the time, or by modern standards.
Back in the 1980s, game designers often had to make a choice between a wide colour template, or higher resolution graphics - some micros of the day offered different resolution modes, so you could have reasonably high-resolution graphics but perhaps only in 2 colours (monochrome), or have lower-resolution graphics (ie bigger 'chunky' looking pixels) but more colours. Because some regions had quite a number of competing micros with varying graphics abilities, games across formats were often compared and as a micro-user during this decade and can recall that terms like "blocky" and "chunky" were often used to suggest that a game on one format had lower resolution than on another.
Sometimes 'chunky' graphics can be a style choice - Minecraft being a good example of a modern game that looks like it is much older. However, in your example, it is referring to an older incarnation of the game and describing it as "chunky" - presumably because of the lower-resolution graphics of the time in which it was made.