Max Texture size of Graphics card not obtained

Hi,
I'm sorry if this came up already. I am having an issue with Texture sizes.
When I load a simple obj (a plane with 25 vertices and a texture assigned in the shader) in the patch of jitter tutorial #36, the texture size seems to be limited to below the specification supported by my graphics card (see below).
If I am using a texture size below 4096, i.e. from 4095 (!) the texture shows up fine, but if I am using bigger sizes, I am getting strange facetting in rainbow colors. No warning or error is printed in the output window.
Neither the actual file size nor the format seem to change anything.
Am I missing something obvious?
Thanks a lot for your help!
Marcel

{
    "Renderer" : "AMD Radeon HD 7800 Series",
    "Vendor Name" : "ATI Technologies Inc.",
    "Version" : "4.2.12002 Compatibility Profile Context 9.12.0.0",
    "GLSL Version" : "4.20",
    "OpenGL Limits" :     {
        "Framebuffers" :         {
            "MAX_COLOR_ATTACHMENTS_EXT" : [ 8 ],
            "MAX_RENDERBUFFER_SIZE_EXT" : [ 16384 ],
            "MAX_VIEWPORT_DIMS" : [ 16384, 16384 ],
            "MAX_DRAW_BUFFERS_ARB" : [ 8 ],
            "SUBPIXEL_BITS" : [ 8 ]
        }
,
        "Points and Lines" :         {
            "ALIASED_LINE_WIDTH_RANGE" : [ 1, 128 ],
            "ALIASED_POINT_SIZE_RANGE" : [ 1, 8192 ],
            "SMOOTH_LINE_WIDTH_GRANULARITY" : [ 0.125 ],
            "SMOOTH_LINE_WIDTH_RANGE" : [ 1, 63 ],
            "SMOOTH_POINT_SIZE_GRANULARITY" : [ 0.125 ],
            "SMOOTH_POINT_SIZE_RANGE" : [ 1, 63 ]
        }
,
        "Stacks" :         {
            "MAX_ATTRIB_STACK_DEPTH" : [ 16 ],
            "MAX_CLIENT_ATTRIB_STACK_DEPTH" : [ 16 ],
            "MAX_COLOR_MATRIX_STACK_DEPTH" : [ 10 ],
            "MAX_MODELVIEW_STACK_DEPTH" : [ 32 ],
            "MAX_NAME_STACK_DEPTH" : [ 64 ],
            "MAX_PROGRAM_MATRIX_STACK_DEPTH_ARB" : [ 32 ],
            "MAX_PROJECTION_STACK_DEPTH" : [ 10 ],
            "MAX_TEXTURE_STACK_DEPTH" : [ 10 ]
        }
,
        "Textures" :         {
            "MAX_3D_TEXTURE_SIZE" : [ 8192 ],
            "MAX_CUBE_MAP_TEXTURE_SIZE" : [ 16384 ],
            "MAX_RECTANGLE_TEXTURE_SIZE_EXT" : [ 16384 ],
            "MAX_TEXTURE_SIZE" : [ 16384 ],
            "MAX_TEXTURE_LOD_BIAS" : [ 16 ],
            "MAX_TEXTURE_MAX_ANISOTROPY_EXT" : [ 16 ],
            "MAX_TEXTURE_UNITS" : [ 8 ]
        }
,
        "Shaders" :         {
            "MAX_COMBINED_TEXTURE_IMAGE_UNITS_ARB" : [ 32 ],
            "MAX_FRAGMENT_UNIFORM_COMPONENTS_ARB" : [ 1024 ],
            "MAX_TEXTURE_COORDS_ARB" : [ 16 ],
            "MAX_TEXTURE_IMAGE_UNITS_ARB" : [ 32 ],
            "MAX_VERTEX_ATTRIBS_ARB" : [ 29 ],
            "MAX_VERTEX_TEXTURE_IMAGE_UNITS_ARB" : [ 32 ],
            "MAX_VERTEX_UNIFORM_COMPONENTS_ARB" : [ 1024 ]
        }

    }

}