Increase size of tiles in MKMapView

Question

I'm currently implementing an MKMapView with a custom tile overlay. Unfortunately the tiles I have are displayed rather small on retina displays making the text on them rather hard to read. Is there a way to scale them up without messing with their location on the map?

I've tried the solution in this question using a custom MKTileOverlayRenderer subclass like this:

class CNTileOverlayRenderer: MKTileOverlayRenderer {
    override func drawMapRect(mapRect: MKMapRect, zoomScale: MKZoomScale, inContext context: CGContext) {

        guard UIScreen.mainScreen().scale > 1.0 else {
            super.drawMapRect(mapRect, zoomScale: zoomScale, inContext: context)
            return
        }

        let tileSize = (self.overlay as! MKTileOverlay).tileSize
        let rect = self.rectForMapRect(mapRect)

        CGContextSaveGState(context)
        let t = CGContextGetCTM(context)
        CGContextConcatCTM(context, CGAffineTransformInvert(t))
        let ratio = tileSize.width / (rect.size.width * 2)

        CGContextTranslateCTM(context, (-rect.origin.x) * ratio, tileSize.height + (ratio * rect.origin.y))
        CGContextScaleCTM(context, ratio, -ratio)

        super.drawMapRect(mapRect, zoomScale: zoomScale, inContext: context)
        CGContextRestoreGState(context)
    }
}

Unfortunately this seems to be messing up my tiles rather badly. I can't say I have a lot if any experience working with CGContexts and only played with some of the parameters without achieving much of a success.