In responsive design "things" (images, text, media, etc) are set to "fill" their container... that way, as the containers adapt to different browser sizes and devices the content is able to adapt.
You simply don't have enough content to fill the space there... will you eventually have enough content? If it's only ever going to be 1 post/article there then I'd recommend a different approach...
You can statically set a minimum height on the container to fill the space but if possible I'd recommend that only be a temporary thing until you have enough content there
on your Block Settings tab for this particular Content Particle, give it custom class like "temp-force-height" or something, then in your custom css:
.temp-force-height .g-content {
min-height: 715px;
}