Nintendo Historical Shipment Data (1983 - Present) (stolen from gaf)

[Augluae]

Here's the comparision that really counts

Year	Gameboy	GBA	DS	3DS
1st	710+380+1160= 2,250M	1060+7570+4610=    11,240M	2120+2190+950= 5,260M	1060+1320+1230= 3,670M
2nd	1300+1540+6430= 9,270M	4920+7800+3770= 14,490M	4780+2920+3720= 11,420M	4790+4670+4060= 13,520M
Third	2990+3910+2770= 9,670M	4080+9450+5000= 18,530M	9120+6630+7810= 23,560M	5690+4270+4000= 13,960M
Total	21,190M	44,260M	40,240M	31,110M

So yeah, in year over year shippment, there's been a tendency to shrink in numbers compared to the GBA and NDS

As for the Wii U, you need at least two years of shippment to make a comparision.