Re: Gal Gadot vs. Lucy Lawless
Lucy Lawless' body shape changed dramatically over the course of the series.
Season 1:
http://www.farfarawaysite.com/section/xena/gallery1/hires/12.jpg
http://www.farfarawaysite.com/section/xena/gallery1/hires/10.jpg
Season 2:
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44284#top_display_media
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44348#top_display_media
Season 3:
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44630#top_display_media
http://www.farfarawaysite.com/section/xena/gallery1/hires/20.jpg
Season 4:
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44463#top_display_media
Season 5:
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44519#top_display_media
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44520#top_display_media
Season 6:
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44465#top_display_media
http://www.farfarawaysite.com/section/xena/gallery2/hires/12.jpg
http://gallery.xenacentral.net/displayimage.php?album=25&pid=44274#top_display_media
During season 1, Lucy Lawless was a full-figured woman, in part because she wasn't eating well during her marriage breakdown (which started around the time she was first cast in Hercules and then continued for the first season).
Towards the end of season 1 and the start of season 2, Lucy Lawless entered into a strict fitness regime to, by her own admission, look more like a "warrior princess" (or rather, her idea of a "warrior princess"), but she suffered multiple injuries, notably a broken hip when she was filming a skit on a horse for a US talk show. Lucy Lawless also suffered a recurring back injury.
So, for late season 2, season 3 and season 4, Lucy Lawless' training was more moderate. Also, in comparison to Gal Gadot, Lucy Lawless' training had to occur outside of the 12 or more hours she spent on set every day filming.
In season 5, Lucy Lawless was pregnant and then dropped an astounding amount of weight very quickly. I think it was around this time that Lucy Lawless decided that pilates was her preferred fitness regime, so her figure in late season 5 and season 6 was probably the slimmest it had ever been during the series.
Also, I think people aren't giving enough credit to the costume. The "Xena" costume starts with undergarments which give "Xena" her impressive bust (and space to hide her breast dagger). The basic costume itself is rather thick leather and the style and cut (including the "V" shaped stitching in the corset) help to emphasise and build upon her womanly figure.
A rather large and cumberson breast-plate (which includes back-armour) is then worn on top of the leather "mini-dress" which adds further bulk to Xena's frame. On top of that, there is the shoulder armour (again, thick leather and metal) and the upper and lower arm bands (also thick leather and metal). Even the boots have a degree of "thickness" about them as they're made to be practical (i.e. they're a pair of Nikes wrapped in leather) and then topped with more "bulk" adding knee pads.
All of that bulk is added to Lucy Lawless' basic frame, which irrespective of the season, is arguably a little broader (and possibly taller) than Gal Gadot. Also, Gal Gadot was a model prior to being cast, which required her to be extremely slim. Lucy Lawless, while a working actress, was first cast back in the 90s when all actors, whether male or female, were generally in more "standard" and healthy shape (as opposed to being stick-thin or overly muscled).
Finally, I have to wonder whether Gal Gadot's costume reveal wasn't photoshopped to ... highlight the purported "bulk" around the upper arms and thighs. It may just be the costume giving that impression, but there are no other photos which seem to indicate that Gal Gadot has quite that level of thickness (at this stage). It's probably what they're working towards, but I'm not sure she is quite there yet.