The problem here is the definition of "fat". If we are comparing everyone to runway model proportions, then yes. However, today there is a large range of what is considered attractive and that's much the way it has always been. What is true of many time periods for European/English fashion is a strong female figure. This means an hourglass shape and proportions, regardless of actual measurements. It's all about ratios, hence why some images of women show what look like incredibly small waists which is actually just large hip ratios.

The very important thing to remember as well is that portraiture is of a real person, often painted because they wanted a portrait done regardless of their level of attractiveness or because the artist thought they represented a character well. Just because the artist thought them attractive or appropriate does not tell us the general public opinion (which is going to vary person to person greatly). It's easier to look at fashions to see what is accented and augmented. Certain time periods like the late 16th century, 1780s, or 1880s accent the backside by way of bum rolls, false rumps, or later bustles. The 1810s have a column shape of gown, but the bust is set very high and round with low necklines. Big sleeves in the 1830s, 1890s, and 1980s? Looks like a smaller waist in comparison (and neck even). Expansion, augmentation, or exposure of certain areas of clothing tells us a lot about a time periods perfect silhouette or focus areas. Cathy Hay, a fashion historian, just did a great post talking about how the extreme silhouettes of the 1900s were created and this is a perfect example of ratio over actual size.