How to reproduce :

• in my skin, a surcharge common-content-body-image-bigsize template in news-main.xsl like this :

  <!-- Illustration big size -->
      <xsl:template name="common-content-body-image-bigsize">
          <xsl:param name="image" select="metadata/illustration/image"/>
          <xsl:param name="alternative" select="metadata/illustration/alt-text"/>
          <xsl:param name="maxWidth" select="'630'"/>
          <xsl:param name="maxHeight" select="'600'"/>
          <xsl:param name="class" select="'big'"/>
          
          <xsl:call-template name="common-content-body-image">
              <xsl:with-param name="image" select="$image"/>
              <xsl:with-param name="alternative" select="$alternative"/>
              <xsl:with-param name="maxWidth" select="$maxWidth"/>
              <xsl:with-param name="maxHeight" select="$maxHeight"/>
              <xsl:with-param name="class" select="$class"/>
          </xsl:call-template>
      </xsl:template>

• then, i create a news with an illustration already optimized and with the right size. The weight of this image is 72ko
• I launch a preview of the news, the quality of the image is different, i download this image and now, its weight is 41ko.