Big enough to take these broken hearts and put 'em in it
The broken hearts line is a metaphor for emotional baggage he carries with him.
The “Broken Hearts” could also reference to the Kanye West album 808’s and Heartbreak. The CD bleeding through the stereo in the trunk.
This line is a precursor to the “pretty good beats on this 808 CD” line from the 2nd verse.
In a more literal sense, Frank has broke a lot of hearts, and now keeps them in his trunk.
The broken hearts can also be interpreted as this: Frank has gotten his heart broken so many times that he could actually fit all of it in the trunk of his car. I say this because of the line “If I feel like a ghost, no Swayze, EVER SINCE I LOST MY BABY”. This last phrase seems to be saying that he has gotten his heart broken again and as such he is going to off himself because he feels dead (i.e. like a ghost) anyway.
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