GB's models are the products of a number of sculptors over the last 15 years. The original ranges were by Duncan and Colin Patten, and more slender than today's models. Duncan did the original civilians, I believe, which are still in production. Duncan's style is very clean and slightly slender. The original Saxons were mostly Duncan's work as well, though many of these have been superceded by newer models.
When Andy and Darren took over the Beast (c.2001 I think), newer sculptors came on board, notably Soapy, who has been the main sculptor since. Soapy's style is slightly more chunky (I really like it) but different to the Pattens. As time went on, and demand for various ranges grew (i.e. when WAB books came out) and moulds wore out, models by Soapy replaced many of the orginal GB minis, notably the Picts. The Welsh, early Saxons and Scots-Irish are all in the same style.
It'd take all day to try to split GB ranges into early/late, and some (like the Vikings) have had some models remastered, resculpted and tweaked so they are a mix of new and old. However, in brief and from memory:
Romano-British/ Age of Arthur, El Cid, Middle Saxons, Vikings, Civilians are mostly (but not all) Patten sculpts from the early years of GB. The 'Ancients' range (Romans, Punic Wars) were scuplted to be compatible with the Foundry (Perry) ranges so are slightly smaller. Most other ranges are more recent and therefore slightly larger.
Hope that helps.