The large lump on the back of the LED, plus cable, plug and socket. Plus driver module on some versions which makes things rather cramped in the headlight bowl, and instructions say to keep wiring away from the heat-sink (the black ribbing at the base):

Both headlights were photographed at the same time and Photoshopped together - LED on the left and halogen on the right (as if you needed me to tell you that). They have the same exposure and aperture as demonstrated by comparing the pilot lights at the bottom ... which look particularly awful with the LED. It shows how the correct alignment is obtained by looking at the edges of the light compared to the lines on the glass. Although the LED is obviously whiter it is barely any brighter as there is relatively little 'burn out' of the lines in the glass compared to the halogen. Also showing the 'dead' area at the top, not present on halogen:

Reflection, with another dead area at the top and just right of centre, although at least that will be facing oncoming vehicles:

In the final analysis even though they are visibly whiter I dispute that they are any better in terms of illuminating things for the driver - I couldn't see any improvement in illuminated distance blocking first one then the other, and with the dead area in the middle may even be worse. Basically it is only the colour temperature that has been changed. And as my insurance company rejected them I returned them for a refund, which was quibble-free.

These from Moss are pretty-much the same despite being a different type. More stuff to get in the bowl:

If you are considering fitting these and your bowls are a bit ratty you might look at Mk1 Mini headlamp bowls. Whereas the MGB ones are 115mm deep the original metal Mini ones are said to be 140mm deep, but check they only have two adjusters and those and the mounting screws to the wing are in the correct place. Note that replacement plastic Mini bowls are the same depth as the MGB. Mini ones may well come with the screw fitting to retain the trim ring which you can't access on the MGB of course. These are replaceable with the MGB ones, but one enterprising chap had fitted the screw before the trim ring, far enough in that the ring clipped over it and was retained, and the rings are much easier to get off than when using the correct MGB spring clips!

But again no brighter only whiter (look at the pattern on the glass) albeit slightly less dead area at the top: