Wellington’s Army Strengths by Year in the Peninsula and in Belgium. In 1809 and 1810 Wellington’s army was small by comparative standards, but steadily increased with the addition of the British-trained Portuguese Army and with attached Spanish units. In 1813 and 1814 Wellington’s army was large, as it was in Belgium in 1815.
The British Army was small, but that was a conscious decision on the part of the British government and the British army in Spain and in Belgium had to be augmented by allied troops to be able to fight the French on anything like an even basis or suffer the fate of Moore’s command in 1808. The French corps and army strengths for 1800 and 1805-1815 are given as a comparison. The average French corps strength in the main theaters are generally consistent from 1805-1815.
The information was taken from Nick Lipscomb’s Peninsular Atlas and the Esposito/Elting Atlas.
British Army strength from 1809-1815:
1809: 27,231.
1810: 34,059 at Torres Vedras.
1811: 54,000 in Estramadura and around Beira under Stewart.
1812: 50,964.
1813: 106,500; at Vittoria 94,000.
1814: 50,000 plus 30,750 Spanish.
1815: 94,600.
Average French Corps Strength by Year:
1800: Less than 20,000 each. Army strength at Marengo: Approximately 28,000.
1805: Less than 30,000 each. Army strength at Austerlitz: 73,200.
1806: Approximately 23,000. Army strength at Jena: Approximately 96,000; Davout’s strength at Auerstadt: 26,000.
1807: Eylau campaign: Approximately 19,000. Army strength at Eylau: 59,600. Friedland campaign: Approximately 21,000. Army strength at Friedland: 80,000.
1808: Approximately 18,500. Army strength for the campaign: 127,300.
1809: Approximately 38,400. Army strength at the outset of the campaign 165,000. Davout’s III Corps initially numbered 67,500. At Wagram in July Davout’s strength was 37,900.
1812: Approximately 31,000. Army strength at the outset of the campaign: 430,400. Borodino: Approximately 21,000. Army strength: 130,000. Davout’s I Corps initially numbered 69,500.
1813: Approximately 20,000 at the outset of the campaign. Army strength at the outset of the campaign: 171,000. Dresden: Approximately 20,000. Army strength on the second day: 120,000.
1814: Approximately less than 20,000. Army strength at the beginning of the campaign: 117,000.
1815: Approximately 20,000. Army strength at the beginning of the campaign: 128,000.

You are absolutely correct. The subject of this thread is the strength of Wellington's army in Spain and in Belgium in comparison to that of the Grande Armee.