Yes they are, quite often.
For example, a Wordpress site that has the popular Gravity Forms plugin will see 70K+ of CSS added to the head by default when GF CSS is enabled (which it is, by default). Most of the time <5K is actually needed. Extrapolate to a few more likely plugins and hey presto, 290K is no problem. Same thing happens with scripts. Each plugin has overhead, and it adds up very fast.