Differences between parts of mixed FEM and single FEM

I am wondering, why I get different sizes and entries in the system matrices when I assemble them
1.) as a submatrix of a mixed FEM ansatz
2.) directly using the same single test/trial spaces

Or do I something fundamentally wrong.

Please consider the following example in python.
For example, the length of the right hand side vector differs when changing the parameter mixed = True, False

from dolfin import *
from scipy.sparse import csr_matrix
import numpy as np

parameters.linear_algebra_backend = "uBLAS"

mixed = False

mesh = UnitSquareMesh(2, 2)

if mixed is True:
 # Define mixed FEM function spaces
 V = VectorFunctionSpace(mesh, "CG", 2)
 Q = FunctionSpace(mesh, "CG", 1)
 W = V * Q

 (u, p) = TrialFunctions(W)
 (v, q) = TestFunctions(W)

else:
 # Define FEM function spaces
 V = VectorFunctionSpace(mesh, "CG", 2)

 u = TrialFunction(V)
 v = TestFunction(V)

# define Form
aa = inner(grad(u), grad(v))*dx

# Assemble system
A = assemble(aa)

# righthandside
fvhomo = Constant((1,2))
Lvh = inner(fvhomo,v)*dx
b = assemble(Lvh)

## Convert DOLFIN representation to numpy arrays
rows, cols, values = A.data()
Aa = csr_matrix((values, cols, rows))
ba = b.array()
ba = ba.reshape(len(ba), 1)

print ba.shape