Keras - error when adding layers to loaded model

Question

I want to use ResNet50 as a feature extractor. For this purpose, I have loaded the pre-trained model, deleted a few layers and added my layers to the model. For adding my layers, I have used the Sequential API. The code is the following:

resnet_model = ResNet50(weights='imagenet')

# Delete layers
for i in range(12):
  resnet_model.layers.pop()

# Fix weights
for layer in resnet_model.layers:
  layer.trainable = False

base_model = Model(inputs=resnet_model.inputs, outputs=resnet_model.layers[-1].output)

model = Sequential()
model.add(base_model)

model.add(Conv2D(2048, kernel_size=(3, 3), input_shape=(7, 7, 2048)))
model.add(BatchNormalization(momentum=bn_momentum))
model.add(Activation('relu'))

model.add(Conv2D(2048, kernel_size=(3, 3)))
model.add(BatchNormalization(momentum=bn_momentum))
model.add(Activation('relu'))

model.add(Flatten())
model.add(Dense(1000, activation='relu'))
model.add(Dense(200, activation='softmax'))

This code works. However, when I run print(model.summary()) I obtain the following:

Model: "sequential_26"
_________________________________________________________________
Layer (type)                 Output Shape              Param #   
=================================================================
model_16 (Model)             (None, 7, 7, 2048)        19115904  
_________________________________________________________________
conv2d_31 (Conv2D)           (None, 5, 5, 2048)        37750784  
_________________________________________________________________
batch_normalization_23 (Batc (None, 5, 5, 2048)        8192      
_________________________________________________________________
activation_917 (Activation)  (None, 5, 5, 2048)        0         
_________________________________________________________________
conv2d_32 (Conv2D)           (None, 3, 3, 2048)        37750784  
_________________________________________________________________
batch_normalization_24 (Batc (None, 3, 3, 2048)        8192      
_________________________________________________________________
activation_918 (Activation)  (None, 3, 3, 2048)        0         
_________________________________________________________________
flatten_10 (Flatten)         (None, 18432)             0         
_________________________________________________________________
dense_17 (Dense)             (None, 1000)              18433000  
_________________________________________________________________
dense_18 (Dense)             (None, 200)               200200    
=================================================================
Total params: 113,267,056
Trainable params: 94,142,960
Non-trainable params: 19,124,096
_________________________________________________________________
None

As you can see, the conv layer adds 37750784 new parameters to the network!!! Obviously, this isn't right, but I can't see why this happens...

Answer 1

Actually, the number of parameters for the second layer is correct, the number of parameters in a convolutional layer is calculated as follows:

n_parameters = n_filters_l * (n_filters_l-1 * (kernel_size_h * kernel_size_w) + 1)
             = 2048 * (2048 * (3 * 3) + 1) = 37750784

If you want to decrease the number of parameters, you therefore have multiple options:

• Decrease the number of filters in the layer before it
• Decrease the number of filters in the layer itself
• Use a lower kernel height of width